This is a demonstration of one of my favorite ab exercises. By using the dumbbell overhead, you are actually working your arms and chest too. You will need a TKO ball and a powerblock (or dumbbell) set to a low-moderately heavy weight.
Sit on the ball with the powerblock (dumbbell) in your lap. Roll the ball back a little, arching your back over it and slowly raise the dumbbell up and lower it behind your head. Lift the dumbbell and crunch up at the same time. The ball will rock back-n-forth with this movement, which is okay. You do not want it to wobble side-to-side. It’s important that you keep a steady balance since this will add an extra challenge to your core.