Aaron Marino of alpha m. demonstrates cable crunches, which takes practice and muscle control to gain benefit.
Use a pulley system and attach handle or rope. Aaron holds handle underhand and then goes down to his knees. He scoots back and brings the cable close to his forehead. Crunch down. Don’t move hands, just upper body. Down, pause, up is the movement. Don’t sit back and pull or use your arms more than you should be.