In order to do ab crunches on a stability ball what you would do is you would lie back on the ball, making sure that your back is pressed into the ball. So you don't want any space between the ball and your back. Your going to reach your hand behind your head, roll back and then your just going to do a slight lift up. So in order to do this crunch correctly you don't want to come up too high, you're not doing a full sit up because then it takes away from some of the ab workout. So what you want to do is you want to just go back and come just a little crunch up. And maybe pause up here, hold it up here. Keeping your neck away from your chin, you want to pretend like you have an apple between your chin and your chest. And then keeping your elbows open. You just breathe on the way up and in on the way down. And then breathe on the way up and on the way down and that's how you would do ab crunches on the ball.