Sometimes Urban Dictionary just cracks me up.
I had reason to google "belly flop" today. Just checking if it was a compound word or two words. Anyway, down the search results was this from UD:
The belly flop is a promiscous act in which the female is getting railed from behind, the male tells her that he is done, whips it out, and blows it on the bed, couch, what have you. As soon as he does so, he sticks his hands between her legs, and knocks her arms and legs out from underneath her, causing her to take a big belly flop.