The abs are actually 4 major muscle groups as listed below......
* Transverse Abdominis (TVA)-The deepest of the abdominal muscles, this lies under the obliques (muscles of your waist). It acts like a weight belt, wrapping around your spine for protection and stability.Target Exercise: Plank
* External Obliques-These muscles are on the side and front of the abdomen, around your waist.Target Exercise: Arm Sweep
* Internal Obliques-These muscles lie under the external obliques, running in the opposite direction.Target Exercise: Crossover Crunch
* Rectus Abdominis-The Rectus Abdominis is a long muscle that extends along the front of the abdomen. This is the 'six-pack' part of the abs that becomes visible with reduced body fat. Target Exercise: Crunch
* Erector Spinae -The erector spinae is actually a collection of three muscles along your neck to your lower back. Target Exercise: Back Extension
http://www.softballperformance.com/blog/wp-content/uploads/image/absbig.jpg
Picture^^^
I do 1 excercise for each of these regions 10-12 reps in circuit fashion. When you can do 3 sets, change the excercises constantly, and if you can use weights. If you haven't noticed Abs are pretty resilent, I am never sore the next day even when I blast them, and I don't even have a cut midsection yet.....
I am working on UD2 right now and have incorporated this ab work into the diet, It is working great, but I still have to get rid of the last layer, I personally don't like to go on the invalidated information (meaning information I haven't proven to be true on myself yet) this workout actually comes from The Abs Diet book, and while it is mainstream, and heavily marketed, people have found their six pack using it. I plan on finding mine =). Hope that is useful. Good luck!