HOW: Place a BOSU ball on the ground with the ball side down. If you need to, hold onto a wall or squat rack for support as you step onto the ball. Make sure your feet are as wide as you can comfortably go with your toes pointed forward. From here, bend your knees, hinge forward at the hips, and squat down and up for the prescribed amount of reps while balancing on the ball.
FEEL: You should feel your leg muscles working.
COMPENSATION: Stay centered on the ball while squatting. Keep your chest up as you squat down.