Knowledge is the key... to shortcuts. I was fortunate to be trained as an instructor (aerobics and fitness) by the best. Knowing certain things allowed me utilize my time and energy to produce the results I wanted.
Depending on the angle at the knees and the type of movement involved, hip flexors are trained. For instance, (straight) leg raises do work the abdominals, but the primary involved are the hip flexors. Full sit ups do the same thing, but the abdominals starts the movement and when the HF ends it. Knowing this, one can easily work the abs by incorporating crunches. Changing the position of your arms will increase/decrease the difficulty/intensity of the exercise. Best exercise, thus far... that I can think of, to work all parts of the rectus abdominis, is crunches with legs straight out. Bring your heals in slightly to bend the knee and, most importantly, protect you back. The difficulty level depends on the arms. Easy: arms crossed at the chest; moderate: hands behind ears, yet elbows flared out of peripheral view; hard: arms extended above head with biceps at the ears; harder: arms across chest hugging a plate (not the ones you eat out of); hardest: plate behind the head; harder than the Mother F**: arms extended with a plate (elbows slightly bent). A 2.5lb will suffice until you've gained enough strength to increase resistance.
When doing laying leg raises, make sure that you bend slightly at the knees and place hands under buttocks to protect the lower back.
Still, your eating and genetics are really important. As for me, I ate healthy, but I was predisposed not to have my pack showing. I was @ 14.5% BF and only the pack was there, but I was happy and healthy.
One thing to consider when doing research is the source. I, too, am no exception. I could be blowing you smoke. Who am I, a lowly 20+ post individual, to give advice? I've probably read the same books you have, saying the same thing (lower and upper abs), but what was the purpose of the publication. Was it to inform by way of selling this information to you or was it through journals and reputable sources.
take care
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I don't buy fitness mags for advice, only for the pictures