The stability ball crunch is an abdominal crunch variation which when performed correctly can provide greater stimulus to the abdominals compared to the traditional crunch (link supplied below).
The key to the ball crunch is to position the ball so that it supports the lower region of the back, allowing the torso to extend down the contour of the ball prior to flexing the torso upward to perform the crunch.
Attention should be given to the size of the stability ball used. Some gym-goers may find the exercise uncomfortable, specifically to the lower back, and may find a smaller ball more suitable due to allowing greater bend in the hip joint.
As with all core exercises, an important point to remember is the myth that surrounds localised fat loss. Abdominal crunches are great for developing the muscles of the abdominals, but they are not going to reduce specific fat levels of the stomach and hips. Instead, an exercise program and diet which promotes overall fat loss should be followed, with exercises such as the ball crunch providing stimulus for the development of the abdominals which will be more notable with overall lower body fat levels.