Pulling chins

From USAFA Folklore

Pulling Chins is an integral part of the brace, or position of attention, expected of basic cadets and fourth class cadets. To pull chins, one must draw the chin back as far as possible into the neck, creating a series of wrinkles in the front of the neck.

Ostensibly, pulling chins is supposed to mimic the position used during ejection from an aircraft, however pulling chins has occasionally been banned by the PTB because of suggestions that it causes nerve damage, or something.