The physioball ball rollout is an anti-extension exercise for your abdominal muscles. As you roll your body forward, your goal is to keep your body and spine in a straight line. It is common to see someone get a large curve in their lower back as they roll forward. We want to avoid this.
How far should you roll out to? If you can keep your body in a straight line, roll as far as you can. The exercise is still effective if you can only roll forward a few inches while maintaining proper body alignment. Your method of progression is to then keep rolling further out while keep your spine and hips in a straight line. You should be trying to tuck your pelvis under you (posterior pelvic tilt) and bracing (squeezing/contracting) your core as you perform the physioball rollouts. This exercise serves the same purpose as the more popular ab rollout wheel. However, for most people, this variation is much easier to perform properly.