This is an excerpt from Back Exercise by Brian Richey.
To activate the spinal stabilizers and entire posterior body to balance on the ball.
Lie face down on the stability ball with the ball under the stomach and hips (a). The hands are on the ground with the arms extended and the feet are on the ground. Press the hips slightly into the ball to stabilize the hips and spine and keep the abdominals engaged (as though to lift the abs inward and away from the ball).
Lift the left leg off the ground by squeezing the left glute and then lift the opposite arm (b). Hold for a moment and then repeat with the other arm and leg. The height of the lift isn’t as important as maintaining stability. When starting this exercise, lift the leg first, then once stable, add the arm. When comfortable, lift both the arm and the leg at the same time.