When you crouch over a sleeping dude's face and punch their testicles, immediatly causing a reflexive situp, so they sit up and their face hits your ass.
Hey, that private over in the LCA just gut atomic sit-upped.
Usually done by frats, you convince someone that they cannot do a situp while you hold a towel over their face (covering their eyes) and hold it strong. they use their strength to try and sit up and while they are doing this a friend pulls his pants down and bends over so his ass is right infront of the person doing the sit up. you then let go of the towel and their face goes straight into your friends ass crack.
little johnny tricked all his friends into doing atomic sit-ups.