This is probably the most advanced Ab exercise there is. Done by Bruce Lee, Sly Stallone and others its an impressive move but killer as well, a few of these reps and you can crunches. This is equal to lots of crunches! I like this one because it helps shape nice firm square ab muscles that show well even when you are not so lean but when down to stage ready leanness it makes for an intense washboard!
Try negatives first......then try reps.