Here's a link to a whole lot of information on the dangers of choking someone out (during sex especially, but also just as a general law enforcement practice):
http://www.jaywiseman.com/SEX_BDSM_BreathPlayMain.php To sum up all kinds of interesting things if you follow that link - breath play (aka choking, erotic asphyxiation, strangulation, choke holds, sleeper holds, knock-out holds, etc.) is very dangerous. Even when it works right, if you knock your partner out, they suffer brain damage. This follows with other sources that I've read that indicate there is NO SAFE UNCONSCIOUSNESS.
I'm often bothered with television treating people like they have an off-switch on the back of their head that you can bop to harmlessly and safely render a character unconscious for the scene. Real life doesn't work remotely that way. It's actually very difficult to knock someone out and have them stay that way, barring narcotics, opiates, alcohol and similar depressive drugs, and barring delirium, extreme fever and similar systemic illnesses. An otherwise healthy, sober person just isn't going to fall down and have a convenient nap whenever the author or script writer desires. If they get hit on the head hard enough to knock them out for more than a few minutes, then they will be *seriously* messed up for weeks or months. Sometimes people NEVER recover from concussions like that. And so when I see that in fic or TV shows, I always cringe.
I recently wrote a fic that involved some very light breath play. Character A presses his throat against character B's hand; no one goes unconscious; at most there is a handful of seconds of "true" obstruction to the airway and no mention of interfering with blood flow. Even with all those stipulations, I'm considering how in-character that was for character A, who has lots of medical training and, at that point in the fic, had no special recuperative powers. There's something to be said for the allure of the forbidden and dangerous. It's something I'll keep in mind.