This is an effective exercise to work your abs and inner thighs. Due to the ball instability, additional stabilizer muscles are involved in the workout.
Lay on the floor with your hands in a push-up position.
Now place your lower shins on the top of the exercise ball.
Keep your body completely straight.
Contract your abs and as you exhale, pull the ball allowing it to roll towards you as close as possible.
Hold this position for 2-3 seconds. And, as you inhale, straighten your legs, rolling the ball back even further so that your arms are inclined towards the floor.
Progress is achieved by improving the coordination and ability to perform this exercise repeatedly, not by increasing the weight.