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.
Betcha cant do an atomic situp
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