Absolutely abs

This is a simple and effective abdominal workout.
Place a small ball between your knees. This is done to isolate the abs and not let the hip flexor muscles assist in the movement. Your thighs also get worked because the adductors are holding and squeezing the ball.

Starting position

Bend the knees. The legs are up. Keep the abs tight and back flat. Point your toes.

Ending position

Maintaining the same angle, lower down the legs and lightly tap or touch the toes on the floor.

Immediately bring the legs back to the starting position.

Repeat this for 30-50 times.

Watchpoints: 

Do not arch the back. 

Let the movement come from your lower abs or hip joint but not from your knees. 

So not stop, hold or rest at the ending position. Immediately get back to the starting position.