This is a great ab exercise. It works really well on a glute ham raise because of the curved pad, but if you can find another piece of equipment to do it on, by all means, get creative!
Loop a band around the back of the glute ham raise. Pull the band over your head so you are holding it at your chest. Rest your middle back on the pad of the GHR. By contracing your abs, crunch yourself forward as far as you can. Slowly return back to the start.
You can increase the resistance by using a thicker band.