Other FReepers have brought up the fact it might have been staged. I was thinking “if that is the way Will hits he’s a tad bit weak for a man”. I would have expected Will to have Rock on the floor after going up there but Rock seemed hardly affected.
I watched the video. It was an moderate open hand slap. If Smith really meant harm, he could break Rock’s jaw or even eye socket. Smith was just defending his wife and didn’t get the connection to Demi Moore’s buzz cut in GI Jane.