Straighten down over the ball, and then use your obliques to crunch up.
However, there's another set of important midsection muscles called obliques, which run diagonally from your ribcage to your hips in two layers, essentially criss-crossing one another to allow for a twisting motion.
There are two sets of diagonal muscles called obliques that criss-cross one another, and work by twisting the torso.
The exercise is lifting your knees straight forward, alternating towards the sides for obliques training.
To hit obliques, place yourself sideways on just one elbow and the side of one foot.