The act of slapping, moving your entire arm in a " slow motion" to seem as if you are in slow motion, when your palm is 5cm away from the subjects face, the speed increases so you slap them hard.
this creates the effect used in the movie 300.
it also looks funny
James: so yeah i was walking dow-
Sai: THIS IS SPARTAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
(Spartan slap)