The expressionfor what you do with your buttcheeks when you need to really need to hold in your fart. Called "nutcracking" because you clench your cheeks so tightly that you could crack a walnut (or some other kind of nut) in between them.
Tim: Hey Loverboy are you ready for some sweet sweet backdoor action now or what?
Bill: I'm sorry Poopykins, but my backdoor is a little musky right now.
Tim: That's OK, I was kind of in the mood for some nutcrackering tonight anyways!