Box Squats

From the article: While descending in the squat, never squat down. Always squat back! If you push the glutes back, the knees will not go forward. In fact, if you sit back far enough, the shins will be past vertical. This is only possible with box squatting

That is how I taught the squat, and it is possible with more than the box squat, can be done with bum to ground.