What Ironfist says is correct. Yeah you can work your abs a trillion reps per day but if your bodyfat is high you will only look more fat. Why? Because you will have developed your abs if you trained for mass and then cover it with a good layer of fat…nobody would know that you have great abs they’d just see a bulge in front…sorta like when you see off season bodybuilders…sure they are bulking but their guts get pretty big, or appear to be because they aren’t cut. Everyone has abs to some degree, others will have low bodyfat and underdeveloped abs perhaps due to metabolism, genetics or whatever, but you don’t get a six-pack directly from doing ab work. You lose body fat to do that…so yeah if you do 100 crunches per day you may burn an extra 400-500 cals which would equate to about a pound a week which will eventually drop enough fat to show your abs. But you could do the same by skipping that big mac meal once a week also.
The ab IS one big muscle and basically cannot be worked region by region. Scientifically speaking, yeah you can work a region of a muscle more than another but not to the point of failure or anything close. When a muscle is stressed the muscle fibres contract…as stress is added more and more fibres are used to lift the load. So technically if you lifted extremely light weight your body wouldn’t recruit 100% of all the muscle fibres to lift the weight…but at that point you are lifting such a small amount of weight you aren’t doing any damage to your muscle so it can rebuild bigger and stronger. So don’t believe in bs like outer and inner pec, or upper and lower abs or biceps…a muscle is either flexed or not…that’s all folks.
Oh and on a last note 2% bodyfat isn’t really possible. Some bodybuilders training for a competition may drop to 4 or 5% for one day only but at 2% you couldn’t sustain it and would simply be dead…but that’s just a trivial matter. 10% bodyfat is quite low so don’t think all those bags of bones out there are like 0%. Just thought I’d add that in.
Oh and to the one that asked if they are wasting their time with an exercise…I would answer that you are never wasting time doing exercise…just because what you do can be done another way doesn’t mean it’s wrong. Do what you are comfortable with.