Stand with your feet shoulder-width apart, with your knees over your heels and all your weight in your heels. If someone were to run into you from various angles, how easy would it be to knock you off balance?
Now stand with your knees driven over your toes, and the weight on the balls of your feel. Now how easy would it be to knock you off balance? This position is a lot more stable! Think about how a boxer stands in the ring, in order to be in a strong, stable, and athletic position.
Watch Simon demonstrate this!