Now, the right way:

Position yourself with your elbows on a standard bench with your lower legs (shins) on a stability ball. Push yourself up into a plank to get into the starting position.

From here, start by rounding your back and tucking in your tailbone. Without shifting your weight forward over the bench, flex your abs to push your low back up towards the ceiling as you allow your knees to bend slightly as they move toward your chest.

It's very important that your focus is on driving your low back up, NOT to actively pull your knees to your chest. This will keep the contraction more in your abs and less in your hip flexors.

Once you're in full flexion, lower your body back to the starting position. Think about extending your spine to neutral, not pushing the legs out. Stop at neutral and repeat.

• Doing this exercise from the floor is a good variation, but it can be too advanced for some lifters. If your intention is work your rectus abdominis, keeping the movement out of the hips is the first thing to consider. Maintaining a set position in the pelvis gives your abs an anchor point to pull on and flex the spine, as well as keeping the legs and hip flexors out of it.
• When you make the movement less advanced, you allow for better control and focus during the whole set. When an exercise is difficult to perform, most of the focus shifts away from the muscles being worked and into completing the movement. This leads to a "get it done, no matter what" kind of thing. The intention is just as important as the movement itself. If you can't feel the muscle working, you're missing the point.
• Perform a slight rocking motion at the bottom of the rep by shifting your weight back toward your feet to insure you keep the focus on your abs and not on your shoulders. This weight transfer into the feet will light up your core.