when the sitter upper is blindfolded and then held down while he trys to do a situp. in the mean time another person puts theyre bare ass in his face and he is let go so is face smashes into the ass.
Dude we gave travis the impossible situp today and he smashed his face into codys ass, GNARRRRRR!!!
When someone (a male) is passed out on their back, you take off your pants and stand with one leg on either side of their torso, with your bare ass in the direction of their face. You then punch them in the groin with a degree of force, and they will sit up abruptly, getting a face full of your bare ass.
Fuck, Josh made me do an impossible situp last night and I swear, I can still taste his ass.