First off, it's called the Atomic Sit-Up. If you call it the "Impossible Sit-Up", you're an idiot, a curse be on your brow. It's also not to be done so close their face touches your butt, it's design is for them to open their eyes and be mooned, leaving at least a foot between their face and your butt. The butt-touching is not funny in the least and reaches a level of homo-eroticism, that I question the people doing it. This is also something that should only be done with good friends. Not a foreign exchange student. Thanks for showing him America.
Secondly, Joe is right.