Punching your hand inside someone's asshole and then you open the asshole and tickle the inside.
The sensation feels like rats crawling up your ass, hence the name.
Nick (you, a kinky motherfucker): "Man, Lisa just gave me rats up my ass."
Mike (your childhood friend who respects you very much, up until this information which has entered the neurons in his brain matter): "What the fuck?"
When someone ate something that their stomach doesn't agree with, this causes pain and more likely than not, a very violent shit follows in suit. Usually diarrhea
I've been eating dairy all day, and now my stomach is in my ass.