Those wishing to build killer abs focus on the rectus muscles. This long flat structure is actually two muscles side by side, originating at the sternum and the bottom ribs, and inserting into the pubic bone.
A thick fascia called the rectus sheath covers the muscle and is held down to the rest of the supporting abdominal structures by tendineous inscriptions. The space between these inscriptions delineate the famed "six pack." The function of this muscle is to move the hip toward the chest. The joint in this case is actually multiple joints – the spine.
There's another muscle group, underneath and below the rectus, which raises the legs toward the chest – the hip flexors. This is not the muscle you want to work if you're intent on working your abs! Unfortunately, I see many people who think they're working their abs when they're predominately working their hip flexors.
Rope crunches are perhaps the most popular ab exercise known to man. If you're kneeling down and bending at the waist, with your back flat, you're not working your abs at all. That's all hip flexor.
To involve your abs, you have to do the exact opposite – lock your hips and bend your back. The start position for a rope crunch is with the spine arched as far back as possible (full extension). It's performed correctly by pulling the elbows toward the knees, with the spine going from full extension to full flexion, while the waist (hips) stay totally locked and stationary. This focuses all the work on the abs.
The same goes for sit-ups, hanging leg raises, and any "crunching" exercise that takes the spine from full extension to full flexion. If you do these right, don't be surprised if you can't use as much weight or do as many reps as you were when you were training your hip flexors.