This variation of the abdominal crunch targets not only the rectus abdominis (the "six pack"), but also the obliques; the muscles that reside on either side of the rectus abdominis. Virtually all ab exercises are executed via a "crunching" motion, whereby the upper torso rolls forward to flex the abdomen. However, this particular exercise is different in that a twisting motion is added to the movement. As you reach with the ball, the torso will naturally rotate and pivot which naturally engages the oblique muscles to a greater degree than the traditional ab crunch.

The following are instructions for how to perform the ab crunch with bilateral reach:

Lie down on your back (supine) with your knees bent. Place feet flat on the floor. Grasp the ball with both hands. Once you are in position, contract/crunch your abdomen by rolling your upper torso forward. At the same time, reach with the ball (arms fully extended) toward the side of the RIGHT knee. Exhale as you reach with the ball. Return to the starting position, then repeat the reaching motion to the side of the LEFT knee. The movement should be performed in alternating fashion. To really get the most out of this exercise, try to hold the ball near the knee for a second or two, then return to the starting position.

In general, this exercise is contraindicated for anyone with lower and/or upper back pathologies.