The "knocked out cold" condition didn't occur to me. To be honest, that sounds awfully powerful for a simple condition. It seems like that's just a step away from the condition "dead."
Dead isn't a condition. Dead requires to gain 4 harm (at least if you are a PC). "Knocked out cold" doesn't seem to be more powerful narratively than any other condition.
My main goal with the custom move was to reward the victim for putting themselves in extreme danger. Getting knocked unconscious by another character (at least in my game) seems dangerous. It's a good set-up for stone-cold murder.
I wanted to give the victim the option--their choice--on whether or not their character falls unconscious. And if they still decide to put their character in that kind of danger, I figured an XP reward was warranted.
In Monsterhearts a 10+ result means the acting player was completely successful and thus gain complete agency. Making the 10+ result about the victim's choice doesn't seem right to me. A 7-9 can be about the victim's choice, but a 10+ should have the player deciding.
Also, consider this: your move is in play all the time. On the one hand, it is a very specific and powerful thing, on the other hand, all the mechanics you need are already in the game. You are not adding anything new. On top of that, now cold becomes a more valuable stat, because it allows you to take three different actions: hold steady, clobbering from behind and shut someone down. That's pretty powerful. Why do I still want volatile as a stat?
The victim's choice won't even matter, if it is an NPC. The MC would go for concussed every time. Then bring it down hard on the attacking character. A PC might go for the XP, but since only PCs make moves, this choice is only useful against another PC. BTW, if I want to be abusive, the PCs could clobber themselves to the season's end in no time. ;)
Next problematic thing is the 7-9 choice. Your move will only result in the desired outcome on a 10+. Everything else will lead to a fight.
In my mind, clobbering someone from behind might require a teenager to hold steady before going through with action. Then they can a string or use another action to gain the string and then expend it.
If you really need a move for this, then try something like this:
To clobber someone from behind, spend a string on them. They go unconscious. If they are a PC, they mark XP and the MC will tell them when and where they wake up.