Drive back up explosively stopping just short of locking your legs out to keep the tension on the quads.
Raise your heels up on a weight plate or block, keeping your heels within 6 inches of each other. Stand tall (A). Sink your hips back and bend at the knees, squatting down until the crease of your hips passes below your knee (B). Drive back up explosively stopping just short of locking your legs out to keep the tension on the quads. Repeat.