When a girl farts and queefs simultaneously, placing one hand "over" her vagina, and the other hand "under" her legs, on her ass, both in a cup-like position. When said girl farts and queefs, she moves her cupped hands from a closed to open position, thus creating a loud "Over-Under Thunder." This act is common when girls have pajama parties.
Cindy: Uh-oh! I have to fart and queef at the same time! What should I do?
Stacey: You should cut an Over-Under Thunder!