This exercise creates a great stretch on the lateral trunk flexors. If you've never done this one before, it can feel a bit awkward until you find the right foot placement against the wall and the right body position over the ball.
Place your top leg about a foot-length behind your bottom leg and keep your back heel slightly off the wall while your front foot is flat against the wall. Also, position your torso over the ball such that you can create a stretch while bending over the ball without feeling like you're going to fall over it.
Lastly, make sure you move over the ball, instead of the ball moving under you.