As you descend into a body weight squat, you shift your weight back onto your heels. This shift helps keep your knees from pushing too far forward over your toes. If you shift your weight forward on the balls of your feet, your knees will push forward past your toes. This places undue stress on your knee joints. Although you should not purposefully push your knees forward, they may naturally move forward even if you execute the squat with perfect form.