What is this called in English I describe the situation as follows:
When a boy goes behind a girl, front-to-back position, especially when the girl is distracted because it makes it funnier to him, hits (kind of smack) her butt using his crotch including the part where his genitals are and then runs away.
Occassionally boys who are close friends do that, too, just to tease eachother. Sometimes that strong that the other one bends backwards somehow or is pushed forward.
Actually it's a kind of sexual harrassment thing if the girl/boy is not OK with that.
The word that I'm looking for doesn't need to include running away, or being funny although if it does that'd be even better. I was thinking of bang someone but I think that's too strong. They're not having sex or anything! Maybe this happens in a disco, too, and the girl either slaps the boy or becomes a date.