Hold a small weight against your chest with one hand and brace the other on something stable in front of you. Rise onto the balls of your feet, then push your knees forward while leaning your torso back, keeping a straight line from your knees through your shoulders rather than folding at the hips. Lower yourself by bending only at the knees, letting your knees travel well past your toes as your torso tilts back, until you feel a deep stretch through the front of your thighs. Drive back up through your knees rather than your hips — there should be almost no hip movement in this one, which is what isolates your quads so directly.