You can (within reason) legally clobber someone in a marking contest, even knocking them out, and all bets are off. It’s always been an anomaly of this game compared to others. And a good thing. Generally you are using this to push off as well, your opponent knows it’s coming.
If he raised something (knee, leg, arm) in one motion or turned his body & stuck his backside out to protect himself, even if it knocked him out then fine. It’s the distinct second action, the only reason for it is to hurt his opponent. Also you don’t need to be looking at someone to whack them.
All players should be doing it every time then, go into a contested ball situation, put your knee out at the last second to protect yourself. Gotta player coming at you from behind/side on, watch where he is when he is about to tackle, then throw the arm back and whack him in the head…sorry last i looked he was standing up and dropped to tackle me, all good though.