As a Lean Warrior, we stopped doing typical machine leg curls a long time ago. We instead utilize bodyweight to build size and strength in your legs. And, Nordic Hamstring Curls are one of the best exercises for your legs.
I unfortunately couldn’t find any images of people performing this exercise with an exercise ball. If you don’t have a lot of strength in your hamstrings, you will find this exercise extremely difficult without an exercise ball.
The purpose of the exercise ball is for stability. You use it to stabilize yourself as you pull your body up with your hamstrings.