very quick post but...SPOILERS!
Ok, so now I've read my way through it completely...
では 刺したのは京楽スタークの影から出てくる
僕の能力は 童（こども）の遊びを現実にする能力
The stabber was Kyouraku, coming out of Stark's shadow.
"My ability is the power to turn childs-play into reality."
I guess now we know why he sees the truth in all things...ne...
Edit to add:
This sounds like a release to me, probably Shun's, relating to the Takaoni technique that he didn't get to do anything with earlier. It's crack, hence my reasoning. But also interesting, too...it seems Shun logic to me...(it's also explaining the game, but...yeah, I may be reading more into it than necessary here).
影鬼なら影を踏まれたら負け 高鬼なら高いほうが勝ち～負ければ死勝てば生～
For the shadow demon, if you're trampled by the shadows, defeat
For the tall demon, the tallest will win If you lose, winning death is life
It seems that this childhood game of Shunsui's is...well...intriguing to say the least. He's stabbed at Stark, which presumably makes Stark a part of the game in some way. He's used what I'm going to say is the 'demon blade' (right hand blade, I think I'm right in thinking that's the longer weapon and the one I tend to call Amaki) since the release relates to demons and so on.
The game of Life and Death:
I wonder now if Katen Kyoukotsu has a subtle meaning relating to this game of Shunsui's. The name, I mean, since there are childhood games with flowers, sticks, bones, etc. Just a thought.
In short, it seems to be that by stabbing at Stark, Shunsui has entered them both into the 'game' of life and death. By calling a "Iro-oni" (maybe it's Iro-ki, not sure without the furigana) and then a colour, an attack is dealt on either party - not just on Stark, but on Shunsui as well if need be. The depth of the attack depends on the amount of colour the attacking individual has on their body.
So Shun starts with grey (haiiro), and he has no grey, so nothing much happens.
Then Stark says white (shiro), and Shun is injured because Stark has a lot of white on his body, obviously. I have no idea if the haori is anything to do with Shun getting hurt here, because this is what threw me before, but I'm going to say...not. Just that he took it off, so...but anyhow, Shun sheds that, and is then all black.
全身黒装束になった京楽が イロ鬼 黒！
とばっさりスタークを斬った所で
Now all dressed in black, Kyouraku calls "Colour Demon: Black!"
Blow by blow it cuts/slashes Stark up."
Clearly...the reason Shun kept trying to slash at Stark in the earlier battle was in order to start the game. So it wasn't so much 'out of character' but him trying to begin his attack. I'm tempted to think of this as his shikai since there's no declaration of bankai nor any change in his sword. It's been mooted that this may only be the power associated with that one blade on the right hand side. My right and left is hopeless so I'm taking other people's word for it, but perhaps the wakazashi has its own power too. We may not get to find out though now since there was a flashback for Stark. I guess that means he died.
Also, Shunsui's strategy of undressing to win just seems apt somehow. Yes, it was to increase the amount of black on his body but...the amused fangirl in me wonders what other games he knows involving the addition or removal of clothing...hrm...
The question remains, however. Where is the Joo? Perhaps sipping tea, taking a nap, waiting for the battle to end...but I'm a little sad that we haven't seen him again yet. I don't really think he'll be in the Halibel fight now - though I suppose anything is possible.
BTW, does this mean Rose and Love together are not as strong as Shunsui? Stark did acknowledge their strength after all - but they weren't able to hurt him and he was ready to destroy them. The way he seems to be acknowledging Shunsui is a different level of acknowledgement. Which suggests that the Senior Taichou are still a cut above the rest. Bring on Retsu vs Yami (please, Kubo, please) and send Juu a get well soon card so we can see him get his own back on Wonderwice!
Credit to Bleach Asylum and Nja @ 2ch for the japanese spoiler text...
Final edit, I promise:
At last I think I get it! *sigh*