Becoming and Royster have it. Your ass has to come back to act as a counterbalance. Otherwise you will fall flat on your face. Just as long as you aren't turning it into a crappy squat you can bend your knees quite a bit. Just make sure you keep the bend in your knees when you come up so that your hams, glutes, and back are doing the weight and not your quads.
Cheers,
Scotsman