This exercise is a simple way to add intensity to your ab workout. Grab a Swiss ball and lie flat on your back. Put your legs up on the Swiss ball so that there is an angle slightly greator than 90° between your pelvis and your legs. This position will make the exercise more intense by shifting the focus of the motion. Now just perform a crunch while maintaining this form. To change it up even more, try doing an alternating crunch (as shown in the picture at the left) to put more emphasis on the obliques (the sides of your abs).

A video demonstration is available for this exercise.
View the Abdominal Workout video to see this exercise along with several others.