I've never really understood the mechanics of that whole taking a slap for someone thing. I'm not sure how one could move in front of a slap done at regular slapping speed. Also, unless they replace the person that's about to be slapped by standing in the exact same position, wouldn't they get hit by the wrist or something instead of the hand? Plus, I think in this case, the senpai is behind that kid as he's about to slap, so short of teleporting, I'm not sure how that'd work.