Visualize this exercise as a sort of “upside down” crunch, as far as the rectus abdominis is concerned. Imagine a rope connecting from your sternum to your pelvis, and there are little guys up there pulling your legs up. This is essentially what you should be doing, you just have to know how to approach it mentally, as with any exercise. Otherwise you are likely using your hips more; think of this more like the opposite of a squat, as far as the hips are concerned. That’s what it sounds to me like you were doing.