Start: Sit at the end of a bench with the balls of your feet on a step, a wooden block, or a couple of phone books about a foot in front of you. Put a towel or sweatshirt over the tops of your knees and rest a barbell across them. Hold it there with your hands. With your torso erect, allow your heels to drop as low as they'll go.
Finish: Press back up to the balls of your feet. Pause then return to the starting position.