person 1 is baited into performing "the hardest situp ever". Person 1 then lies on the floor on their back and told cross hands on chest. Person 2 then holds a towel over the face of person 1 covering the eyes telling him sit up quickly with all his strength when he say to. Meanwhile person 3 has removed their pants and is squating over person 1's face with out him knowing. Person 2 then tells person 1 to sit up quickly and removes the towel as soon as the sit up begins. The result is person 1's nose is planted into person 2's ass.
blindfolded, someone tries to do a sit up while holding the hands of someone else so they cant. little do they kno someone is bending over with thier ass out so when the person doing the situp lets go, they get a face full of browneye.
"Oh man, we gave jake teh BEST atomic sit up last night!"
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.
when one is conned into attempting a "most difficult" situp resulting in a firm thrusting of the suckers nose directly up the shit pipe of a prankster. Usually results in prompt ass kicking and residual smell of butt mud for several days.
See the movie Heavy Weights or any highschool locker room