Works your lower back and abdominals
Adding a Swiss (or Stability Ball as I call it) to any excerise can up the toughness level and makes you work those abs even harder
Result - nice shoulders for your summer tank top, plus good posture
We do this move in P90X. We see how many we can get in like 30 seconds. You definitely feel the burn.
Helps strengthen your total core (including your back muscles)