(Meta) Slayer Abilities
I've been thinking about Slayer powers: how they work, what their limitations are, and what the experience of using her powers would feel like to the Slayer in question. Hence this essay, which is the first long meta I've written for quite some time.
First, let's address the elephant in the room: the show did not always show Slayer abilities in a consistent fashion. The meta-explanation for this is that the writers didn't want to restrict their opportunities for future story-telling; once something has been defined narrowly in black-and-white, you can't easily change it even if you then have a really great idea for a new episode. Joss famously once said that the starship in 'Firefly' travels "at the speed of plot". Of course, sometimes the inconsistencies are less about a deliberate artistic choice and more about continuity errors and simple mistakes.
However, this still works even if we look at it purely within the context of the Buffyverse. If Slayer powers aren't always shown consistently on the screen, that can simply reflect the fact that they don't always work consistently in-world either. They are, after all, magical in origin; and it's implicit in the definition of magic that it doesn't obey the normal rules of physics or cause & effect.
Buffy tells Kendra that her emotions make her stronger, give her power. We know that when she's feeling uncertain, depressed, bewildered or insecure, her fighting ability suffers ('Seeing Red' is the classic example, along with 'The Freshman'.) More interestingly, when she suffers a near-death experience (or, being Buffy, an actual-death experience), she often comes back stronger than ever ('Prophecy Girl', 'Bad Girls').
My hypothesis is therefore that the Slayer Power lies within every Slayer, and she can draw on it to perform superhuman feats of strength and skill. However, she can't necessarily do so automatically and completely; many Slayers can only access a proportion of their total potential. Doubt, confusion and uncertainty limit a Slayer's ability to use the power; a Slayer who is at one with herself, focussed and attuned to her feelings can do so much more easily because her subconscious mind is not limiting her access to it.
It's often suggested that there is a darkness within Slayers: it's what Dracula tells Buffy in 5.01. Of course, we later learn that their power is demonic in origin. This could be why near-death experiences help Buffy to become stronger, and why anger feeds her power; the greater connection to death and slaughter brings her closer into tune with the Slayer power inside her. However, it doesn't necessarily follow that a Slayer who is overwhelmed by hatred and rage is going to be better at tapping her power than one who isn't. The idea that the Slayer essence is dark, demonic and evil is one that's expressed by various people in the show but never established authoritatively: and indeed, it's contradicted by the First Slayer herself when Buffy seeks to understand the true nature of her power:
"You are full of love. You love with all of your soul. It's brighter than the fire ... blinding. That's why you pull away from it. Love is pain, and the Slayer forges strength from pain. Love, give, forgive. Risk the pain. It is your nature."
I would suggest, then, that the Slayer essence is something primal, and frightening in its intensity, and certainly dangerous and predatory; but it's not a force of hatred, and such emotions don't help you get closer to it. Faith in S3 isn't a stronger Slayer than Buffy. A Slayer motivated by righteous anger and a desire to protect her loved ones is more in tune with the Slayer power than one driven by merely the urge to hurt and destroy.
There's also the matter of training. At first, one might question why Slayers need to train at all, if their power is supernatural in origin. Certainly Buffy's superstrength doesn't rest in the size of her muscles; it's not physical in origin. I doubt that examining a Slayer's body by conventional medical means - or even dissecting one, or running it through a gene sequencer - would show it to be the slightest bit different to that of a normal human. Slayer abilities come from magic, not from working out regularly.
So why train? I suggest that a big part of it is to help a Slayer learn to trust her powers, and draw on them instinctively and without thinking. She has to become one with herself. I think it's very significant that when Buffy asks Giles for 'advanced training' in S5, he has her doing things like yoga - focussing on her inner spiritual development as much as conditioning her body. Even in the real world, regular training helps build self-confidence and discipline; given the nature of a Slayer's power, I suggest that for them such qualities have supernatural as well as psychological value.
On a more practical level, however, there's value in being physically fit and in good condition even if you have superstrength. I've proposed that a Slayer's abilities are inconsistent and not universally reliable. If Buffy's having an off day or is too upset to tap into her mystical powers to the full extent, it can help a lot that she's also a fully proficient martial artist and gymnast in her own right, who's been training on an intensive basis almost every day since she was 15. Not to mention, of course, that fighting techniques are an entirely different matter to physical abilities: it doesn't matter how strong Buffy is if she telegraphs her punches or leaves her guard down.
(On a related note, as I've mentioned before: someone like Kennedy in S7 who was brought up by a Watcher would have been training constantly in combat skills ever since she was a small child. Even without supernatural powers, that would be enough for her to qualify as some sort of superhero or ninja in many non-Buffyverse mythologies.)
So that's what I think about the fundamental nature of a Slayer's powers. Now let's look in more detail at some of their specific abilities and how they work in practice. We all know the basic list: superstrength, fast reflexes, Slayer healing ability and prophetic dreams. Some others are only mentioned in passing, or hinted at rather than described explicitly, or might be Buffy Summers powers rather than Slayer powers as such: intuition into a enemy's weaknesses, aptitude with all kinds of weapons, memories of past Slayers, and so forth.
Superstrength
This is the best-known Slayer power, the one most often referred to on the show. Its precise limits have never been laid down in text - something I gather is quite deliberate on the writers' part, but we can make some guesses. A Slayer's main use for her strength is, of course, in punching and kicking man-sized opponents across the room. We've seen Buffy bending apart iron bars or the barrel of a rifle. In 'Life Serial' she easily picks up a steel girder - which, I gather, normally weigh around 100 - 120 kg, or coincidentally about the weight of a large man. In 'Anne' she struggles to lift a large iron gate, which I'm guessing would probably weigh around 200 - 250 kg (based on looking up websites of companies selling metal gates for driveways and the like.) She's never been seen picking up motor cars and tossing them at opponents. She and Spike together did manage to demolish a house, but it took them quite some time - not like Glory who could achieve similar results with a single stamp of her foot.
In short, under normal circumstances a Slayer's strength is more on a par with the very strongest non-supernatural humans than with Marvel or DC costumed superheroes. (The Olympic weightlifting record is currently 263 kg, which I've suggested is roughly in line with what Buffy struggles to lift in 'Anne'). That's not to say that a Slayer might not be capable of bursts of greater strength if the story required it for some dramatic scene - it can be justified as the power inside her temporarily being unleashed fully - but a normal Slayer still isn't going to be tossing steam locomotives around.
Something of note is that we rarely see Slayers using superstrength for extended periods of time. It's always in short bursts, to hit someone or bend something or maybe pick up something heavy. Anya actually calls this out in 'The Replacement' when she asks why they can't load Buffy up "like one of those little horses" to help move all their furniture into their new apartment at once. It's possible that Buffy could, in fact, do this if she wanted to, but doesn't because it would be undignified. Alternatively, it may be in the nature of the Slayer power that it's designed for fighting, not extended feats of strength, and acts more like an adrenaline rush that gives a temporary boost.
In fact, there's some evidence that Slayer strength is not "always on" at all. In 'A New Man' when Giles become a supernaturally strong Fyarl demon, he goes around accidentally smashing things and demolishing walls. Buffy never does that, despite the fact that she's often shown as being quite clumsy. It might be that she's just got used to her strength and knows how to limit it; or alternatively it could be that her superstrength only manifests when she's in danger or when she consciously draws on it.
A popular theme in some fanfic is a new Slayer acquiring superstrength and accidentally breaking someone's back or crushing them because she doesn't know what she's doing - however, we've never seen anything like that happen on the TV show. In the movie, Buffy was surprised by the fact that she was suddenly much stronger, but she didn't accidentally kill anyone or even cause any non-deliberate property damage with her new powers. Counter-evidence is that in Season 8, Andrew's 'recruitment video' does show a Slayer accidentally breaking a vase and complaining that she "can't control her strength", and in S6 there's a scene where Giles complains that Buffy's hug is too tight because she's "still remarkably strong".
However, the second example needn't refer to Slayer strength at all; given all the martial arts training Buffy does, she's going to be "remarkably strong" for a woman her size regardless of any powers she might have. The first is more difficult to explain, although the fact that this is Andrew's idea of what becoming a Slayer is like might be enough in itself. Of course, even if a new Slayer doesn't automatically use her superstrength in everyday life, she might draw on her powers to do something she would have struggled with as a normal human without realising exactly how much stronger she now is.
There's also the issue of sex, which for some reason is perennially popular among fic writers. *g* If a Slayer's superstrength is always working rather than being drawn on as needed, then if she's thrashing around in the grip of an uncontrollable and instinctive climax she's likely to do some serious damage to her partner. Not to mention that orgasmic vaginal contractions from someone capable of bending a steel bar in half are likely to be rather dramatic in their effects on anything that's inside her at that moment... But since Riley and Parker, not to mention Faith's many normal-human casual boyfriends, don't appear to be maimed for life after sleeping with a Slayer, we can presume that Slayers don't necessarily use their superstrength even when they've lost conscious control of their actions. Which isn't to say that a Slayer couldn't deliberately choose to use her superstrength during sex, or that if she felt suddenly endangered it might kick in anyway. (Ouch.)
Because a Slayer's strength is supernatural and not dependent on the actual strength of her body, it's likely to manifest in unexpected ways when she uses it. If a 50 kg girl does a jump-kick on a 100 kg man powerful enough to send him flying across the room, the laws of physics should demand that she go flying backwards herself at twice the speed. It doesn't seem to work that way with Slayers: their strength seems to appear from nowhere, and affect the world without affecting them in return. Newton's Third Law need not apply. What this means in a fight is that a Slayer can deliver punches and kicks with her full strength without having to bother about things like angles or leverage or bracing herself against the ground. Fighting her will be a bewildering experience for someone more used to fighting natural opponents, or even demons who are superstrong due to their muscles rather than magic. (Note: credit goes to, um, someone? Maybe eowyn_315? for originally suggesting this particular aspect of a Slayer's powers. I didn't think of it myself.)
Running and Jumping
I mention this separately although it's really a subset of superstrength. We saw in 'The Harvest' that Buffy can do a standing jump over a perimeter fence which is probably just over two metres high. Given that the world record for high jump is 2.45 m, this is again in line with her strength, making a Slayer's powers roughly equivalent to the maximum possible normal human abilities.
As for running, the shooting script of 'Two To Go' mentions her running at "40 miles per hour", which is about the speed of a racehorse at full gallop. That's much faster than the world record for a human runner (45 kph or 28 mph), which seems inconsistent with the level of her other powers. On the other hand, the 40 mph figure has never been corroborated on screen or in a canonical comic, so we could ignore it: or alternatively, use this as an example of a Slayer in dire extremis and emotional trauma being able to tap into her powers more fully and exceed her normal limits. Given my suggestion above that Slayer strength is more useful in short adrenaline bursts than sustained long-term use, it's also likely that Slayers would be better at sprinting than long distance running - though their stamina also comes into play there.
Buffy also shows excellent gymnastic abilities - think of the training montage in 'Once More With Feeling' for example, or her ice skating talents in 'What's My Line' - but it's unclear if that's Slayer abilities or Buffy's own natural talent. She does tell Angel that she was heavily into skating even before being Chosen. (Of course in the real world, this talent of Buffy's reflects Sarah Michelle Gellar's own proficiency at skating, which she wanted to use on the show.) Plus, she was also a cheerleader before becoming a Slayer. Buffy is naturally athletic.
Super Reflexes
Most of the time, Slayers appear to move not much faster than a normal human; they don't go all blurry with superspeed or do the thing where they move faster than the eye can follow and suddenly appear in front of someone. On the show, Glory did have that power and Buffy couldn't counter it; I seem to remember Angel doing it once too, but I can't place the reference - and besides, if vampires in the Buffyverse could do that it would seem a fatal shortcoming if the Slayer couldn't match and exceed that power.
On the other hand, Buffy does sometimes perform superhuman feats of reflex and reaction. She catches a crossbow bolt in mid-flight in 'Help', for example (something we've also seen vampires do, of course). It doesn't seem that she can dodge bullets, but she can move faster than a person shooting at her can move the barrel of their gun to aim it - see 'Angel' and 'Primeval'. That does imply that Slayers can move at supernaturally fast speeds, but only in short bursts at moments of supreme danger, just as I suggested for their strength.
However, there's an alternative explanation, which is that Slayers' reflexes aren't due to superspeed but rather to danger sense or a kind of intuitive, instinctive precognition. (I discuss this more below.) By that reading, a Slayer is able to catch a crossbow bolt because her hand starts moving before the bolt has even been fired. She doesn't dodge at supernatural speed, she's already moved away from where her opponent was going to hit her before his brain has finished telling his fist to move. However, because this ability relies on intuition and the Slayer's unreliable precognitive powers, it doesn't always work, which is why Buffy often ends up getting hit anyway. Or even if she subconsciously knows in advance her opponent is going to throw a particular punch, she might be moving in that direction already and simply be unable to dodge in time.
Stamina
This isn't an ability that's referred to all that often, and might simply be a reflection of Buffy's purely human level of physical fitness; but while we do see her out of breath or exhausted after a difficult fight sometimes ('Faith, Hope and Trick' against Kakistos, for example), she does seem to have more stamina than a normal person. Parker makes a joke about it in a sexual context, in fact, but that might be just him being crude. In 'Wolves At The Gate' Buffy and Xander both start running across a field at the same time; not only is Buffy about twice as fast as him, but when they reach their destination he's bent double, sweating and panting for breath, and she's maybe breathing a little more heavily than normal (the artist has drawn her with her mouth open, that's the only way to tell.)
Slayer metabolism seems to work much more efficiently than a normal human's. Buffy eats like a pig (according to Dawn) after slaying, but there's not a gram of spare fat on her body by S7. That suggests that overweight Slayers may be a rarity even if they don't train regularly like Buffy (or her Slayer Army in S8) are shown to do. Whether it works the other way, and Slayers actually need to eat more food than a normal person to sustain themselves, is unclear. Doug Petrie does mention in his commentary to 'The Initiative' that he wanted to show a scene with Buffy eating about three times the usual amount of food for her normal lunch, but that idea was vetoed. And given that Buffy's idea of binge eating is "craving a low fat yoghurt", it could be that when Dawn describes her sister as "such a pig" she's using Hollywood actress terms of reference rather than anything connected to the real world.
Slayer Healing
"Slayers heal fast. Real fast" as the bartender tells Warren in 'Villains'. Exactly how fast seems to be rather inconsistent, however. In 'Fool For Love', Buffy suffers a major stab wound to the abdomen then gets thrown hard against a crypt. The next morning, the wound is still so painful that she passes out when Riley bandages it; by the following evening, it hurts if Spike punches her right on the wound; by the next episode, it's completely healed. When Faith suffered remarkably similar injuries in 'Graduation Day' she ended up in a coma for eight months. Broken bones seem to heal within 24 hours ('Killed By Death'); Buffy also recovered completely from a serious fever in the same space of time. Healing is not, however, instantaneous; Slayers can of course be killed easily enough.
Whether Slayers can be scarred by their injuries is an open question. Fanfic often suggests that scars heal completely, leaving no mark at all. Certainly Buffy doesn't seem to be particularly scarred up from her many fights - although there's inconsistency here because Wishverse!Buffy did have a big facial scar. It's possible, of course, that it was recent and if you saw her in a week's time the scar would have gone. Buffy's bite mark from 'Graduation Day' is also treated inconsistently; it's not always visible, but characters in S4 and even S5 notice it and comment on it. Maybe it doesn't fade because it was supernatural in origin? (The same fanwank is often suggested for why Spike never healed his eyebrow scar from Xin Rong's sword.)
Slayers still feel pain, but it seems that either it's much less than normal humans, or (like vampires) they don't let it slow them down.
Slayer healing doesn't render a Slayer immune to disease, though as we saw in 'Killed By Death' it does allow them to recover incredibly quickly, and maybe lessens the intensity of the symptoms. I think it would be a reasonable supposition, though there's nothing in canon to confirm or deny it, that Slayers tend to be very healthy in general. An interesting supposition would be that vaccination doesn't work on a Slayer because her body's supernatural defences are too efficient in dealing with the small amount of pathogens present - but exposure to the full-scale disease would have an effect on her similar to vaccination of a normal human. Infect a Slayer with smallpox or TB, and she'll suffer a slight rash and fever for a day, then be immune to the disease...
Can Slayers be poisoned? Giles drugs Buffy in 'Helpless', but that's a specially-tailored and possibly mystic cocktail of drugs specifically developed by the Watchers' Council to affect Slayers. Buffy gets out of her head drunk in 'Beer Bad', but again that's not normal alcohol but supernaturally-potent beer. My suggestion would be, again, that Slayers can be poisoned but they're much more resistant than normal people. If a Slayer has strength equal to an Olympic weightlifter and speed equal to an Olympic sprinter, then she has toughness equal to Grigori Rasputin.
A final interesting question; what about long-term injuries and physical impairments? Are there any disabled Slayers, or do their supernatural healing abilities allow them to recover even from things that would permanently cripple a normal person? Can they regrow nerve tissue, or even entire organs? Canon sources are silent, except through omission - we've never seen a Slayer with a permanent disability that I can remember. Given the general principles I've established above, I suggest that a Slayer's long-term healing powers are roughly equal to the maximum possible that's been observed in humans. There are rare cases of spontaneous remission of supposedly 'incurable' conditions, and maybe in Slayers such things are routine; but the spontaneous regrowth of major organs is never possible.
Supernatural Senses
Many fanfic stories assume that Slayers have extremely acute senses, especially sight and hearing. It's a reasonable assumption to make: a predator needs to be able to spot their prey instantly, and it's confirmed in canon many times that vampires have supernaturally good hearing and sense of smell. If her natural opponents can spot the Slayer coming long before she knows they're there, her effectiveness will be greatly reduced. Unfortunately, I'm not aware of anything in canon which confirms that Slayers can see or hear better than normal people.
Buffy does sometimes show remarkable situational awareness - hitting Giles on the head with a dodgeball even when blindfolded, for instance, or reacting to danger as someone sneaks up behind her. However, that could be due to mystic danger sense or simlar powers rather than anything so mundane as being able to hear Giles's breathing or whatever. That said, I do prefer the idea that Slayers have very acute senses for my own fic - but that's acute for a human, not supernaturally potent.
Aptitude with Weapons
This is something that’s suggested rather than being explicitly spelled out; that Slayers have a natural affinity for weapons. It's best illustrated in 1.07 'Angel' where Giles assumes Buffy will need "countless hours of vigorous training" to learn how to fight with a quarterstaff, but she masters it in seconds. Buffy also shows herself to be something of a weapons geek, whether it's critiquing the fight scenes in a kung fu movie, discussing the combat uses of a bastinado with Principal Wood, or decorating her bedroom in Scotland with a display of antique weaponry. However, that does suggest that this may at least in part be a specifically Buffy thing rather than a general Slayer power.
Danger Sense and Tactical Ability
As mentioned above, Slayers are proficient at fighting even when blindfolded. They can react to danger almost before it happens (or actually before it happens, possibly). Sneak up behind a Slayer and swing an axe at her head, and there's a good chance you'll find she's grabbed your arm to prevent you finishing the blow without having given any prior sign she even knew you were there. This might be due to very good hearing or whatever, but my preferred hypothesis is that it's a mystical ability. Slayers often have prophetic dreams, so there's already something in their abilities that allows them to see into the future, if not always clearly or reliably. Being able to predict danger a moment or two before it happens seems like a logical extension of that talent that would be invaluable to a supernatural demon-fighter.
I don't think it's a conscious ability, nor is it always reliable. Buffy didn't dodge Warren's bullet, after all, even though she did stop the crossbow bolt aimed at Cassie's head. A reasonable conjecture is that even if a Slayer feels a premonition of danger, she might ignore it if she has other things on her mind, or hasn't learned to trust her instincts. The source of peril might also make a difference, with Slayers being more attuned to supernatural opponents than to a normal man with a gun.
A related talent is Buffy's apparent ability to sense when a normal-seeming person is actually evil, even when her friends dismiss her as being paranoid: for example Ted, or Kathy in 'Living Conditions'. This doesn't appear to be a specific demon-detection (or robot-detection) ability - she doesn't realise Angel is a vampire until he vamps out on her, for example; but it could be explained as part of her danger sense. Or it could simply be that Buffy is a very intuitive person who trusts her instincts, and not supernatural at all.
One thing that needs to be mentioned in this context is the supposed Slayer ability to detect vampires by getting cramps in the lower abdomen whenever one comes near. That was in the film, but never mentioned once in the TV show; I get the impression Joss was rather embarrassed by it all by then. In 'Welcome To The Hellmouth' Giles does tell Buffy that she should be able to sense the presence of vampires mystically, but by 'reaching out with her mind and letting the energy wash over her' rather than anything involving period pain. And anyway, Buffy prefers to detect vampires through their bad fashion sense than by anything mystical; as we've seen, she doesn't sense that Angel is a vampire at first. It's possible though, that other Slayers - and maybe Buffy herself even if she doesn't like using it - do have this mystical talent.
Because a Slayer can sense danger before it happens, that gives her an advantage in combat beyond the basic physical level of punching and dodging. Buffy often shows remarkable situational awareness, being able to use her environment to the maximum advantage in a fight, be it by swinging around a pole for extra speed or making an improvised weapon out of whatever happens to be around. Melaka Fray describes the experience of fighting as a Slayer like this: "I'm slick with power and I feel the fight as it changes... as it flows... everything into place, perfect, and I finally do what I was born to do. I slay." That hyper-awareness of what's happening, of how the fight is developing, of what her opponents are about to do, seems to be key to a Slayer's combat prowess. It's not "bullet time", to use the computer gaming term - Slayers don't have superspeed - but I do suggest that fighting will often feel to a Slayer as if time's slowed down, and everyone around her except herself is moving in slow motion, because she's already sensing what they're about to do and can react to it much faster than they can.
What this means is that a Slayer might not necessarily be good at formal tactical planning - though she's likely to develop a lot of experience in that line over time if she survives - but she'll be brilliant at on-the-spot improvisation. Simply walking into the middle of the bad guys' lair might appear to be bad tactics from a conventional military perspective, but it's actually playing to a Slayer's greatest strength.
Finally, Buffy in particular seems to have a knack of finding an opponent's weak spot. It may take her a couple of tries - in fact, she seems to fall into a standard pattern when fighting tough opponents of losing the first battle, then coming back for the second round and beating them. "I always find a way" - if staking a turok-han doesn't work, she'll twist its head off with a chain instead. If the dragon has a chink in its armour over its breast, Buffy will be the one to spot it. That may, again, be just her particular talent, or it may be linked to the Slayer's supernatural tactical skill.
Leadership Ability
Buffy is a natural leader, charismatic and inspirational. (Most of the time anyway; not always.) Even other Slayers seem to acknowledge this, most obviously in Season 8. However, several of the other characters suggest that she's the leader because she's the Slayer, and not just because she's the best fighter. Motivating people and leading them against the forces of darkness seems to be one of the expected duties of the Slayer, and this at least implies that some of her supernatural gifts are intended to make her a better leader. Indeed, before it came to mean "leadership ability" the word charisma meant "a gift of divine grace" - what you'd expect to find in a Chosen One.
Prophetic Dreams
These were depicted as a key part of the Slayer's abilities in the early seasons, although prophetic dreams - as opposed to dreams showing Buffy's psychological turmoil - were less prominent in later seasons (although they made something of a come-back in Season 7). Sometimes Buffy's dreams gave her extra insight into current situations, such as in 'Innocence' where a dream told her of Jenny's involvement in Angel losing his soul, or in 'Beneath You' where her dream warned her that Potentials were being hunted down and killed. However, in some cases they definitely foretold the future, either directly ("Are you sure you're ready for this, Buffy?" in 'Surprise') or obscurely ("Oh yeah, miles to go. Little Miss Muffit counting down from 7-3-0.")
Of course, the dreams are not controllable in any way - despite Xander's wishes, Buffy can't dream of lottery numbers. It's also not clear if they're an inherent Slayer power, or if they're being sent to her by some outside force - similar to the visions sent by the Powers That Be on 'Angel'. Given that they're generally referred to as 'Slayer Dreams' and no mention is ever made on 'Buffy' of anyone directing the dreams, it seems likely that the first is true. Maybe the original demon used by the Shadowmen to create the First Slayer had powers of prophecy, and some of its abilities stuck.
I've used the idea in a few of my own stories that after 'Chosen', when there are 2,000 or so Slayers in the world, then sometimes every one of them will have the same prophetic dream simultaneously. It seems like a logical development - although it's likely each of them would remember slightly different things from the dream once they wake up.
Slayer Heritage
Slayers can access memories of the lives and histories of past Slayers. These can manifest as vivid dreams or nightmares, but the information also seems to be available to them - at least on an instinctive if not conscious level - while awake. It helps to explain their natural combat talent and ability to spot weaknesses in opponents: they may not have fought them before in person, but they have the combined experience of eight thousand years of past Slayers to draw on.
This power is an unusual one to discuss, because it was very frequently mentioned in many canonical Buffyverse works, but not the actual 'Buffy' TV show! (Which leads some people to reject its existence.) It's in the movie, where newly-called Buffy was having persistent nightmares about the lives of past Slayers. It's in 'Fray', where it's a major plot point that Melaka doesn't have these dreams or memories although she should. It's in 'Angel', where Dana is reliving the lives of past Slayers and in her madness is unable to separate their experiences from her own. It's in Season 8, where the unnamed Slayer of 'The Chain' is seen being hit by a vision of past Slayers, and Melaka refers to it as a reason why Buffy has an unfair advantage in fighting her. But as far as I can remember, it's not in Seasons 1 - 7 of 'Buffy'.
An interesting detail is that in the original shooting script of 'Chosen', Buffy refers to the Scythe as containing "the energy and history of so many Slayers", shortly after the last of the Guardians told her that it was more than just a weapon. I've speculated that the Slayer memories were not part of the original package of powers intended by the Shadowmen; after all, they'd want to keep the Slayers ignorant so they would be a tool in their hands, and letting them have access to the life experiences of all their predecessors seems to be directly counter-productive to that. But what if it were the Scythe - forged by the Guardians, opponents of the Shadowmen - which was recording and storing the Slayer heritage and somehow making it available to new Slayers? Slowly, generation by generation, that would eat away at the total control over their Slayers that the Shadowmen had intended, giving those women knowledge and information and, ultimately, the power to take control of their own lives again. It's a nice metaphor.
And on that note, I’ll end this essay. I may have missed some of the Slayer's powers, but I think this is quite long enough already. :-)
First, let's address the elephant in the room: the show did not always show Slayer abilities in a consistent fashion. The meta-explanation for this is that the writers didn't want to restrict their opportunities for future story-telling; once something has been defined narrowly in black-and-white, you can't easily change it even if you then have a really great idea for a new episode. Joss famously once said that the starship in 'Firefly' travels "at the speed of plot". Of course, sometimes the inconsistencies are less about a deliberate artistic choice and more about continuity errors and simple mistakes.
However, this still works even if we look at it purely within the context of the Buffyverse. If Slayer powers aren't always shown consistently on the screen, that can simply reflect the fact that they don't always work consistently in-world either. They are, after all, magical in origin; and it's implicit in the definition of magic that it doesn't obey the normal rules of physics or cause & effect.
Buffy tells Kendra that her emotions make her stronger, give her power. We know that when she's feeling uncertain, depressed, bewildered or insecure, her fighting ability suffers ('Seeing Red' is the classic example, along with 'The Freshman'.) More interestingly, when she suffers a near-death experience (or, being Buffy, an actual-death experience), she often comes back stronger than ever ('Prophecy Girl', 'Bad Girls').
My hypothesis is therefore that the Slayer Power lies within every Slayer, and she can draw on it to perform superhuman feats of strength and skill. However, she can't necessarily do so automatically and completely; many Slayers can only access a proportion of their total potential. Doubt, confusion and uncertainty limit a Slayer's ability to use the power; a Slayer who is at one with herself, focussed and attuned to her feelings can do so much more easily because her subconscious mind is not limiting her access to it.
It's often suggested that there is a darkness within Slayers: it's what Dracula tells Buffy in 5.01. Of course, we later learn that their power is demonic in origin. This could be why near-death experiences help Buffy to become stronger, and why anger feeds her power; the greater connection to death and slaughter brings her closer into tune with the Slayer power inside her. However, it doesn't necessarily follow that a Slayer who is overwhelmed by hatred and rage is going to be better at tapping her power than one who isn't. The idea that the Slayer essence is dark, demonic and evil is one that's expressed by various people in the show but never established authoritatively: and indeed, it's contradicted by the First Slayer herself when Buffy seeks to understand the true nature of her power:
"You are full of love. You love with all of your soul. It's brighter than the fire ... blinding. That's why you pull away from it. Love is pain, and the Slayer forges strength from pain. Love, give, forgive. Risk the pain. It is your nature."
I would suggest, then, that the Slayer essence is something primal, and frightening in its intensity, and certainly dangerous and predatory; but it's not a force of hatred, and such emotions don't help you get closer to it. Faith in S3 isn't a stronger Slayer than Buffy. A Slayer motivated by righteous anger and a desire to protect her loved ones is more in tune with the Slayer power than one driven by merely the urge to hurt and destroy.
There's also the matter of training. At first, one might question why Slayers need to train at all, if their power is supernatural in origin. Certainly Buffy's superstrength doesn't rest in the size of her muscles; it's not physical in origin. I doubt that examining a Slayer's body by conventional medical means - or even dissecting one, or running it through a gene sequencer - would show it to be the slightest bit different to that of a normal human. Slayer abilities come from magic, not from working out regularly.
So why train? I suggest that a big part of it is to help a Slayer learn to trust her powers, and draw on them instinctively and without thinking. She has to become one with herself. I think it's very significant that when Buffy asks Giles for 'advanced training' in S5, he has her doing things like yoga - focussing on her inner spiritual development as much as conditioning her body. Even in the real world, regular training helps build self-confidence and discipline; given the nature of a Slayer's power, I suggest that for them such qualities have supernatural as well as psychological value.
On a more practical level, however, there's value in being physically fit and in good condition even if you have superstrength. I've proposed that a Slayer's abilities are inconsistent and not universally reliable. If Buffy's having an off day or is too upset to tap into her mystical powers to the full extent, it can help a lot that she's also a fully proficient martial artist and gymnast in her own right, who's been training on an intensive basis almost every day since she was 15. Not to mention, of course, that fighting techniques are an entirely different matter to physical abilities: it doesn't matter how strong Buffy is if she telegraphs her punches or leaves her guard down.
(On a related note, as I've mentioned before: someone like Kennedy in S7 who was brought up by a Watcher would have been training constantly in combat skills ever since she was a small child. Even without supernatural powers, that would be enough for her to qualify as some sort of superhero or ninja in many non-Buffyverse mythologies.)
So that's what I think about the fundamental nature of a Slayer's powers. Now let's look in more detail at some of their specific abilities and how they work in practice. We all know the basic list: superstrength, fast reflexes, Slayer healing ability and prophetic dreams. Some others are only mentioned in passing, or hinted at rather than described explicitly, or might be Buffy Summers powers rather than Slayer powers as such: intuition into a enemy's weaknesses, aptitude with all kinds of weapons, memories of past Slayers, and so forth.
Superstrength
This is the best-known Slayer power, the one most often referred to on the show. Its precise limits have never been laid down in text - something I gather is quite deliberate on the writers' part, but we can make some guesses. A Slayer's main use for her strength is, of course, in punching and kicking man-sized opponents across the room. We've seen Buffy bending apart iron bars or the barrel of a rifle. In 'Life Serial' she easily picks up a steel girder - which, I gather, normally weigh around 100 - 120 kg, or coincidentally about the weight of a large man. In 'Anne' she struggles to lift a large iron gate, which I'm guessing would probably weigh around 200 - 250 kg (based on looking up websites of companies selling metal gates for driveways and the like.) She's never been seen picking up motor cars and tossing them at opponents. She and Spike together did manage to demolish a house, but it took them quite some time - not like Glory who could achieve similar results with a single stamp of her foot.
In short, under normal circumstances a Slayer's strength is more on a par with the very strongest non-supernatural humans than with Marvel or DC costumed superheroes. (The Olympic weightlifting record is currently 263 kg, which I've suggested is roughly in line with what Buffy struggles to lift in 'Anne'). That's not to say that a Slayer might not be capable of bursts of greater strength if the story required it for some dramatic scene - it can be justified as the power inside her temporarily being unleashed fully - but a normal Slayer still isn't going to be tossing steam locomotives around.
Something of note is that we rarely see Slayers using superstrength for extended periods of time. It's always in short bursts, to hit someone or bend something or maybe pick up something heavy. Anya actually calls this out in 'The Replacement' when she asks why they can't load Buffy up "like one of those little horses" to help move all their furniture into their new apartment at once. It's possible that Buffy could, in fact, do this if she wanted to, but doesn't because it would be undignified. Alternatively, it may be in the nature of the Slayer power that it's designed for fighting, not extended feats of strength, and acts more like an adrenaline rush that gives a temporary boost.
In fact, there's some evidence that Slayer strength is not "always on" at all. In 'A New Man' when Giles become a supernaturally strong Fyarl demon, he goes around accidentally smashing things and demolishing walls. Buffy never does that, despite the fact that she's often shown as being quite clumsy. It might be that she's just got used to her strength and knows how to limit it; or alternatively it could be that her superstrength only manifests when she's in danger or when she consciously draws on it.
A popular theme in some fanfic is a new Slayer acquiring superstrength and accidentally breaking someone's back or crushing them because she doesn't know what she's doing - however, we've never seen anything like that happen on the TV show. In the movie, Buffy was surprised by the fact that she was suddenly much stronger, but she didn't accidentally kill anyone or even cause any non-deliberate property damage with her new powers. Counter-evidence is that in Season 8, Andrew's 'recruitment video' does show a Slayer accidentally breaking a vase and complaining that she "can't control her strength", and in S6 there's a scene where Giles complains that Buffy's hug is too tight because she's "still remarkably strong".
However, the second example needn't refer to Slayer strength at all; given all the martial arts training Buffy does, she's going to be "remarkably strong" for a woman her size regardless of any powers she might have. The first is more difficult to explain, although the fact that this is Andrew's idea of what becoming a Slayer is like might be enough in itself. Of course, even if a new Slayer doesn't automatically use her superstrength in everyday life, she might draw on her powers to do something she would have struggled with as a normal human without realising exactly how much stronger she now is.
There's also the issue of sex, which for some reason is perennially popular among fic writers. *g* If a Slayer's superstrength is always working rather than being drawn on as needed, then if she's thrashing around in the grip of an uncontrollable and instinctive climax she's likely to do some serious damage to her partner. Not to mention that orgasmic vaginal contractions from someone capable of bending a steel bar in half are likely to be rather dramatic in their effects on anything that's inside her at that moment... But since Riley and Parker, not to mention Faith's many normal-human casual boyfriends, don't appear to be maimed for life after sleeping with a Slayer, we can presume that Slayers don't necessarily use their superstrength even when they've lost conscious control of their actions. Which isn't to say that a Slayer couldn't deliberately choose to use her superstrength during sex, or that if she felt suddenly endangered it might kick in anyway. (Ouch.)
Because a Slayer's strength is supernatural and not dependent on the actual strength of her body, it's likely to manifest in unexpected ways when she uses it. If a 50 kg girl does a jump-kick on a 100 kg man powerful enough to send him flying across the room, the laws of physics should demand that she go flying backwards herself at twice the speed. It doesn't seem to work that way with Slayers: their strength seems to appear from nowhere, and affect the world without affecting them in return. Newton's Third Law need not apply. What this means in a fight is that a Slayer can deliver punches and kicks with her full strength without having to bother about things like angles or leverage or bracing herself against the ground. Fighting her will be a bewildering experience for someone more used to fighting natural opponents, or even demons who are superstrong due to their muscles rather than magic. (Note: credit goes to, um, someone? Maybe eowyn_315? for originally suggesting this particular aspect of a Slayer's powers. I didn't think of it myself.)
Running and Jumping
I mention this separately although it's really a subset of superstrength. We saw in 'The Harvest' that Buffy can do a standing jump over a perimeter fence which is probably just over two metres high. Given that the world record for high jump is 2.45 m, this is again in line with her strength, making a Slayer's powers roughly equivalent to the maximum possible normal human abilities.
As for running, the shooting script of 'Two To Go' mentions her running at "40 miles per hour", which is about the speed of a racehorse at full gallop. That's much faster than the world record for a human runner (45 kph or 28 mph), which seems inconsistent with the level of her other powers. On the other hand, the 40 mph figure has never been corroborated on screen or in a canonical comic, so we could ignore it: or alternatively, use this as an example of a Slayer in dire extremis and emotional trauma being able to tap into her powers more fully and exceed her normal limits. Given my suggestion above that Slayer strength is more useful in short adrenaline bursts than sustained long-term use, it's also likely that Slayers would be better at sprinting than long distance running - though their stamina also comes into play there.
Buffy also shows excellent gymnastic abilities - think of the training montage in 'Once More With Feeling' for example, or her ice skating talents in 'What's My Line' - but it's unclear if that's Slayer abilities or Buffy's own natural talent. She does tell Angel that she was heavily into skating even before being Chosen. (Of course in the real world, this talent of Buffy's reflects Sarah Michelle Gellar's own proficiency at skating, which she wanted to use on the show.) Plus, she was also a cheerleader before becoming a Slayer. Buffy is naturally athletic.
Super Reflexes
Most of the time, Slayers appear to move not much faster than a normal human; they don't go all blurry with superspeed or do the thing where they move faster than the eye can follow and suddenly appear in front of someone. On the show, Glory did have that power and Buffy couldn't counter it; I seem to remember Angel doing it once too, but I can't place the reference - and besides, if vampires in the Buffyverse could do that it would seem a fatal shortcoming if the Slayer couldn't match and exceed that power.
On the other hand, Buffy does sometimes perform superhuman feats of reflex and reaction. She catches a crossbow bolt in mid-flight in 'Help', for example (something we've also seen vampires do, of course). It doesn't seem that she can dodge bullets, but she can move faster than a person shooting at her can move the barrel of their gun to aim it - see 'Angel' and 'Primeval'. That does imply that Slayers can move at supernaturally fast speeds, but only in short bursts at moments of supreme danger, just as I suggested for their strength.
However, there's an alternative explanation, which is that Slayers' reflexes aren't due to superspeed but rather to danger sense or a kind of intuitive, instinctive precognition. (I discuss this more below.) By that reading, a Slayer is able to catch a crossbow bolt because her hand starts moving before the bolt has even been fired. She doesn't dodge at supernatural speed, she's already moved away from where her opponent was going to hit her before his brain has finished telling his fist to move. However, because this ability relies on intuition and the Slayer's unreliable precognitive powers, it doesn't always work, which is why Buffy often ends up getting hit anyway. Or even if she subconsciously knows in advance her opponent is going to throw a particular punch, she might be moving in that direction already and simply be unable to dodge in time.
Stamina
This isn't an ability that's referred to all that often, and might simply be a reflection of Buffy's purely human level of physical fitness; but while we do see her out of breath or exhausted after a difficult fight sometimes ('Faith, Hope and Trick' against Kakistos, for example), she does seem to have more stamina than a normal person. Parker makes a joke about it in a sexual context, in fact, but that might be just him being crude. In 'Wolves At The Gate' Buffy and Xander both start running across a field at the same time; not only is Buffy about twice as fast as him, but when they reach their destination he's bent double, sweating and panting for breath, and she's maybe breathing a little more heavily than normal (the artist has drawn her with her mouth open, that's the only way to tell.)
Slayer metabolism seems to work much more efficiently than a normal human's. Buffy eats like a pig (according to Dawn) after slaying, but there's not a gram of spare fat on her body by S7. That suggests that overweight Slayers may be a rarity even if they don't train regularly like Buffy (or her Slayer Army in S8) are shown to do. Whether it works the other way, and Slayers actually need to eat more food than a normal person to sustain themselves, is unclear. Doug Petrie does mention in his commentary to 'The Initiative' that he wanted to show a scene with Buffy eating about three times the usual amount of food for her normal lunch, but that idea was vetoed. And given that Buffy's idea of binge eating is "craving a low fat yoghurt", it could be that when Dawn describes her sister as "such a pig" she's using Hollywood actress terms of reference rather than anything connected to the real world.
Slayer Healing
"Slayers heal fast. Real fast" as the bartender tells Warren in 'Villains'. Exactly how fast seems to be rather inconsistent, however. In 'Fool For Love', Buffy suffers a major stab wound to the abdomen then gets thrown hard against a crypt. The next morning, the wound is still so painful that she passes out when Riley bandages it; by the following evening, it hurts if Spike punches her right on the wound; by the next episode, it's completely healed. When Faith suffered remarkably similar injuries in 'Graduation Day' she ended up in a coma for eight months. Broken bones seem to heal within 24 hours ('Killed By Death'); Buffy also recovered completely from a serious fever in the same space of time. Healing is not, however, instantaneous; Slayers can of course be killed easily enough.
Whether Slayers can be scarred by their injuries is an open question. Fanfic often suggests that scars heal completely, leaving no mark at all. Certainly Buffy doesn't seem to be particularly scarred up from her many fights - although there's inconsistency here because Wishverse!Buffy did have a big facial scar. It's possible, of course, that it was recent and if you saw her in a week's time the scar would have gone. Buffy's bite mark from 'Graduation Day' is also treated inconsistently; it's not always visible, but characters in S4 and even S5 notice it and comment on it. Maybe it doesn't fade because it was supernatural in origin? (The same fanwank is often suggested for why Spike never healed his eyebrow scar from Xin Rong's sword.)
Slayers still feel pain, but it seems that either it's much less than normal humans, or (like vampires) they don't let it slow them down.
Slayer healing doesn't render a Slayer immune to disease, though as we saw in 'Killed By Death' it does allow them to recover incredibly quickly, and maybe lessens the intensity of the symptoms. I think it would be a reasonable supposition, though there's nothing in canon to confirm or deny it, that Slayers tend to be very healthy in general. An interesting supposition would be that vaccination doesn't work on a Slayer because her body's supernatural defences are too efficient in dealing with the small amount of pathogens present - but exposure to the full-scale disease would have an effect on her similar to vaccination of a normal human. Infect a Slayer with smallpox or TB, and she'll suffer a slight rash and fever for a day, then be immune to the disease...
Can Slayers be poisoned? Giles drugs Buffy in 'Helpless', but that's a specially-tailored and possibly mystic cocktail of drugs specifically developed by the Watchers' Council to affect Slayers. Buffy gets out of her head drunk in 'Beer Bad', but again that's not normal alcohol but supernaturally-potent beer. My suggestion would be, again, that Slayers can be poisoned but they're much more resistant than normal people. If a Slayer has strength equal to an Olympic weightlifter and speed equal to an Olympic sprinter, then she has toughness equal to Grigori Rasputin.
A final interesting question; what about long-term injuries and physical impairments? Are there any disabled Slayers, or do their supernatural healing abilities allow them to recover even from things that would permanently cripple a normal person? Can they regrow nerve tissue, or even entire organs? Canon sources are silent, except through omission - we've never seen a Slayer with a permanent disability that I can remember. Given the general principles I've established above, I suggest that a Slayer's long-term healing powers are roughly equal to the maximum possible that's been observed in humans. There are rare cases of spontaneous remission of supposedly 'incurable' conditions, and maybe in Slayers such things are routine; but the spontaneous regrowth of major organs is never possible.
Supernatural Senses
Many fanfic stories assume that Slayers have extremely acute senses, especially sight and hearing. It's a reasonable assumption to make: a predator needs to be able to spot their prey instantly, and it's confirmed in canon many times that vampires have supernaturally good hearing and sense of smell. If her natural opponents can spot the Slayer coming long before she knows they're there, her effectiveness will be greatly reduced. Unfortunately, I'm not aware of anything in canon which confirms that Slayers can see or hear better than normal people.
Buffy does sometimes show remarkable situational awareness - hitting Giles on the head with a dodgeball even when blindfolded, for instance, or reacting to danger as someone sneaks up behind her. However, that could be due to mystic danger sense or simlar powers rather than anything so mundane as being able to hear Giles's breathing or whatever. That said, I do prefer the idea that Slayers have very acute senses for my own fic - but that's acute for a human, not supernaturally potent.
Aptitude with Weapons
This is something that’s suggested rather than being explicitly spelled out; that Slayers have a natural affinity for weapons. It's best illustrated in 1.07 'Angel' where Giles assumes Buffy will need "countless hours of vigorous training" to learn how to fight with a quarterstaff, but she masters it in seconds. Buffy also shows herself to be something of a weapons geek, whether it's critiquing the fight scenes in a kung fu movie, discussing the combat uses of a bastinado with Principal Wood, or decorating her bedroom in Scotland with a display of antique weaponry. However, that does suggest that this may at least in part be a specifically Buffy thing rather than a general Slayer power.
Danger Sense and Tactical Ability
As mentioned above, Slayers are proficient at fighting even when blindfolded. They can react to danger almost before it happens (or actually before it happens, possibly). Sneak up behind a Slayer and swing an axe at her head, and there's a good chance you'll find she's grabbed your arm to prevent you finishing the blow without having given any prior sign she even knew you were there. This might be due to very good hearing or whatever, but my preferred hypothesis is that it's a mystical ability. Slayers often have prophetic dreams, so there's already something in their abilities that allows them to see into the future, if not always clearly or reliably. Being able to predict danger a moment or two before it happens seems like a logical extension of that talent that would be invaluable to a supernatural demon-fighter.
I don't think it's a conscious ability, nor is it always reliable. Buffy didn't dodge Warren's bullet, after all, even though she did stop the crossbow bolt aimed at Cassie's head. A reasonable conjecture is that even if a Slayer feels a premonition of danger, she might ignore it if she has other things on her mind, or hasn't learned to trust her instincts. The source of peril might also make a difference, with Slayers being more attuned to supernatural opponents than to a normal man with a gun.
A related talent is Buffy's apparent ability to sense when a normal-seeming person is actually evil, even when her friends dismiss her as being paranoid: for example Ted, or Kathy in 'Living Conditions'. This doesn't appear to be a specific demon-detection (or robot-detection) ability - she doesn't realise Angel is a vampire until he vamps out on her, for example; but it could be explained as part of her danger sense. Or it could simply be that Buffy is a very intuitive person who trusts her instincts, and not supernatural at all.
One thing that needs to be mentioned in this context is the supposed Slayer ability to detect vampires by getting cramps in the lower abdomen whenever one comes near. That was in the film, but never mentioned once in the TV show; I get the impression Joss was rather embarrassed by it all by then. In 'Welcome To The Hellmouth' Giles does tell Buffy that she should be able to sense the presence of vampires mystically, but by 'reaching out with her mind and letting the energy wash over her' rather than anything involving period pain. And anyway, Buffy prefers to detect vampires through their bad fashion sense than by anything mystical; as we've seen, she doesn't sense that Angel is a vampire at first. It's possible though, that other Slayers - and maybe Buffy herself even if she doesn't like using it - do have this mystical talent.
Because a Slayer can sense danger before it happens, that gives her an advantage in combat beyond the basic physical level of punching and dodging. Buffy often shows remarkable situational awareness, being able to use her environment to the maximum advantage in a fight, be it by swinging around a pole for extra speed or making an improvised weapon out of whatever happens to be around. Melaka Fray describes the experience of fighting as a Slayer like this: "I'm slick with power and I feel the fight as it changes... as it flows... everything into place, perfect, and I finally do what I was born to do. I slay." That hyper-awareness of what's happening, of how the fight is developing, of what her opponents are about to do, seems to be key to a Slayer's combat prowess. It's not "bullet time", to use the computer gaming term - Slayers don't have superspeed - but I do suggest that fighting will often feel to a Slayer as if time's slowed down, and everyone around her except herself is moving in slow motion, because she's already sensing what they're about to do and can react to it much faster than they can.
What this means is that a Slayer might not necessarily be good at formal tactical planning - though she's likely to develop a lot of experience in that line over time if she survives - but she'll be brilliant at on-the-spot improvisation. Simply walking into the middle of the bad guys' lair might appear to be bad tactics from a conventional military perspective, but it's actually playing to a Slayer's greatest strength.
Finally, Buffy in particular seems to have a knack of finding an opponent's weak spot. It may take her a couple of tries - in fact, she seems to fall into a standard pattern when fighting tough opponents of losing the first battle, then coming back for the second round and beating them. "I always find a way" - if staking a turok-han doesn't work, she'll twist its head off with a chain instead. If the dragon has a chink in its armour over its breast, Buffy will be the one to spot it. That may, again, be just her particular talent, or it may be linked to the Slayer's supernatural tactical skill.
Leadership Ability
Buffy is a natural leader, charismatic and inspirational. (Most of the time anyway; not always.) Even other Slayers seem to acknowledge this, most obviously in Season 8. However, several of the other characters suggest that she's the leader because she's the Slayer, and not just because she's the best fighter. Motivating people and leading them against the forces of darkness seems to be one of the expected duties of the Slayer, and this at least implies that some of her supernatural gifts are intended to make her a better leader. Indeed, before it came to mean "leadership ability" the word charisma meant "a gift of divine grace" - what you'd expect to find in a Chosen One.
Prophetic Dreams
These were depicted as a key part of the Slayer's abilities in the early seasons, although prophetic dreams - as opposed to dreams showing Buffy's psychological turmoil - were less prominent in later seasons (although they made something of a come-back in Season 7). Sometimes Buffy's dreams gave her extra insight into current situations, such as in 'Innocence' where a dream told her of Jenny's involvement in Angel losing his soul, or in 'Beneath You' where her dream warned her that Potentials were being hunted down and killed. However, in some cases they definitely foretold the future, either directly ("Are you sure you're ready for this, Buffy?" in 'Surprise') or obscurely ("Oh yeah, miles to go. Little Miss Muffit counting down from 7-3-0.")
Of course, the dreams are not controllable in any way - despite Xander's wishes, Buffy can't dream of lottery numbers. It's also not clear if they're an inherent Slayer power, or if they're being sent to her by some outside force - similar to the visions sent by the Powers That Be on 'Angel'. Given that they're generally referred to as 'Slayer Dreams' and no mention is ever made on 'Buffy' of anyone directing the dreams, it seems likely that the first is true. Maybe the original demon used by the Shadowmen to create the First Slayer had powers of prophecy, and some of its abilities stuck.
I've used the idea in a few of my own stories that after 'Chosen', when there are 2,000 or so Slayers in the world, then sometimes every one of them will have the same prophetic dream simultaneously. It seems like a logical development - although it's likely each of them would remember slightly different things from the dream once they wake up.
Slayer Heritage
Slayers can access memories of the lives and histories of past Slayers. These can manifest as vivid dreams or nightmares, but the information also seems to be available to them - at least on an instinctive if not conscious level - while awake. It helps to explain their natural combat talent and ability to spot weaknesses in opponents: they may not have fought them before in person, but they have the combined experience of eight thousand years of past Slayers to draw on.
This power is an unusual one to discuss, because it was very frequently mentioned in many canonical Buffyverse works, but not the actual 'Buffy' TV show! (Which leads some people to reject its existence.) It's in the movie, where newly-called Buffy was having persistent nightmares about the lives of past Slayers. It's in 'Fray', where it's a major plot point that Melaka doesn't have these dreams or memories although she should. It's in 'Angel', where Dana is reliving the lives of past Slayers and in her madness is unable to separate their experiences from her own. It's in Season 8, where the unnamed Slayer of 'The Chain' is seen being hit by a vision of past Slayers, and Melaka refers to it as a reason why Buffy has an unfair advantage in fighting her. But as far as I can remember, it's not in Seasons 1 - 7 of 'Buffy'.
An interesting detail is that in the original shooting script of 'Chosen', Buffy refers to the Scythe as containing "the energy and history of so many Slayers", shortly after the last of the Guardians told her that it was more than just a weapon. I've speculated that the Slayer memories were not part of the original package of powers intended by the Shadowmen; after all, they'd want to keep the Slayers ignorant so they would be a tool in their hands, and letting them have access to the life experiences of all their predecessors seems to be directly counter-productive to that. But what if it were the Scythe - forged by the Guardians, opponents of the Shadowmen - which was recording and storing the Slayer heritage and somehow making it available to new Slayers? Slowly, generation by generation, that would eat away at the total control over their Slayers that the Shadowmen had intended, giving those women knowledge and information and, ultimately, the power to take control of their own lives again. It's a nice metaphor.
And on that note, I’ll end this essay. I may have missed some of the Slayer's powers, but I think this is quite long enough already. :-)