When working the core it is nice to add some resistance.  In this exercise Amer Delic is performing a crunch on the Swiss Ball with an added db to his chest.

The added weight will create more resistance and will work his upper abdominal cavity with increased difficulty.

Place a towel underneath the db if you find that it is a bit heavy on your chest.

Also, keep your lower back flat on the ball.  This will ensure that you do not put pressure on your lower discs.

For more great exercises check back soon!