Hanging Knee Raise
This also works the hip flexors as well as abdominals. The slings should come all the way up to your armpits, and your hands should simply be resting on the straps. Raise your knees as high as you can, then lower them. Best to have a partner with one arm raised in front of your chest as a target to touch with your knees, and one hand behind your back to keep you from swinging. Try alternating upward twists to each side to work the obliques. As your grip and pulling strength improves, try to do these without the slings, hanging from the bar by your hands instead.