Behold the "Janda Sit-Up"
You'll be shocked at how difficult this is. The idea is to flex your hamstrings against the pull of the cable, which minimizes the degree which your hip flexors can assist the Sit-Up (vis reciprocal inhibition). The heavier the weight on the cable the harder it is. If you can do 5 sets, increase the weight. This drill is to increase your core strength. Good Luck!