Starting Position: Lie prone (on your stomach) over the top of a properly-inflated stability ball (one that compresses approximately 6" under your body weight) with both feet and hands on the floor, hip- and shoulder-width apart.
Outward Phase: Gently exhale, contract your abdominal/core muscles to stiffen your torso and slowly walk yourself forward, lifting your legs off the floor. Continue walking out until the fronts of your thighs or knees are resting on the top of the ball. Maintain a rigid torso so that your legs and trunk form one continuous line. Retract and depress your scapulae (pull your shoulder back and down) as your reach your end position, with your arms fully extended and hands positioned directly under your shoulders. Hold this position briefly.
Return Phase: Inhale and slowly walk yourself backwards to your starting position.
Exercise Variation (1): To further increase the balance challenge, walk yourself further out until the tops of your feet or toes rest on the top of the ball.
Exercise Variation (2): To further increase the balance challenge, raise one leg off the ball while in the static press (walkout) position.