How do I perform a ball back extension?

Lie face down on a stability ball, with the ball placed under your stomach.  Straighten our legs, keep your feet hip-width apart and toes pointed down at the floor. Place your arms across your chest and bend forward at the waist. Draw-in your navel, contract your glutes and tuck-in our chin. Keep your legs straight as your raise your upper body until it is in line with your legs. Return to the starting position and repeat.

Start
1. Lie with ball under belly, feet pointed toward floor, legs straight and shoulder width apart.
2. Cross arms on chest.

Movement
3. Bend forward at waist.
4. Draw-in bellybutton, squeeze butt muscles and tuck chin.
5. Keep legs straight - raise upper body until in line with legs.
6. Return to start.