When a girl rubs her ass all over a guys penis. For the girl, even if she's doing all the work it still makes her wet and makes her want it to go on forever. For the guy, he just stands and enjoys. It's a win, win situation besides the fact that it never last long enough.
Tiffany grinded her ass all over Aidan's penis till he got a boner, Tiffany and Aidan both enjoyed grinding each other.