I am getting my abdominals into rock-hard, punchproof shape by following what is called the Hollywood Abs Training System.
I'm doing exercises like the butterfly crunch, reverse butterfly crunch, vertical ergonomic crunches and a variation with a twisting crunch added. It's like doing one big triset with each movement taken to absolute failure.
The only trouble is that I am feeling the butterfly and reverse crunches in my hips more than my abs. Other than that I think the program is great for reducing the fatty overhang.
What's the skinny on the crunches anyway?
When performing any of the trunk and hip-rolling motions that you describe (crunches etc.), the most important factor involved is to keep the small of the back in constant contact with the bench or floor you are lying on. As soon as you arch your back, you will notice that the stress on the rectus is relieved, and the work load is shifted to the psoas muscles.
These psoas muscles, by the way, attach to the upper thigh bones and both sides of the spine. Their function is to pull the upper body to a sitting position. In the leg raises or vertical ergonomic crunches as you call them they pull the legs up. Also the psoas muscles help to stabilize the hips and trunk when a weight is held overhead or upon the shoulders.
The rectus abdominus has a very simple function. This muscle attaches to the pelvic basin and the sternum and shortens the distance between these two points.
Abdominal work, contrary to popular opinion, does not produce localized or spot reducing. Only nutrition produces fat loss. So do not labor under the delusion that abdominal work will cause a subcutaneous fat loss. However, minimum rest between sets is a must because rapid heart and lung action is very important to emulsify fat.
My observation in the gym is that very few bodybuilders ever break a sweat. They sometimes rest as long as five minutes between sets. This type of workout tempo is too slow to produce definition.
Its a good thing you are not doing full situps because this seemingly deceptive abdominal exercise is not a rectus abdominal exercise at all. Its function is to help stabilize the trunk by static contraction. The psoas major, minor, and quadratus lumborum perform the work by pulling the body up.