Atomic Situp: The act of convincing someone that he/she cannot do a sit-up with the prankster holding a towel over his/her head and pulling forcefully in the opposite direction. Another friend puts their exposed ass by the knees of the person doing the sit-up. The prankster then lets go of the towel which forces the person doing the sit-up face first into the rear of the third friend.
http://media.ebaumsworld.com/wmv/atomicsitup.wmv
http://media.ebaumsworld.com/wmv/atomicsitup.wmv
That's plain evil