Lie face down with ball under your hips. Bend your left knee and lift up towards your chest. Finish by extending and driving your left leg behind you. Return to the starting position and repeat.
Here are a few suggested alternative exercises:Ab CrunchAb Crunch - MachineAb Crunch ObliquesBall Crunch TraditionalDouble CrunchDumbbell Side BendsKneeling Cable CrunchKneeling Oblique TwistLateral Bend with Barbell