This is for core stability. Get on your knees behind the ball. Place your forearms on the ball, your back should be naturally arched and your abdominal pulled in. Slowly roll the ball forward, keeping your abs braced, extending the arms out straight, and not allowing the back to collapse. If you begin to feel it in your back you have either gone too far or lost control of the abs. Use your core to pull the ball back to the start position.