In response to your first question, I'm no expert on ab form but I try as much as possible to pull my navel in toward my spine when doing ab work, traditional or otherwise.
As far as Pilates flattening my abs more than traditional ab work, here's my take on what works best for me. (You know, of course, that everybody's different, though):
My abs are my biggest trouble spot and I have tried for YEARS to strengthen and flatten them. That's counting before and after giving birth to 3 children. Even with regular, consistent ab work (mostly from the Firm), I could barely get my shoulder blades off the floor. The first time I noticed significant improvement was when I started doing the Intensities and yoga in 2002. Doing Bootcamp once a week, in particular, seems to be a big piece of the "ab puzzle" for me. I've also found that doing abs 6 days a week, rather than alternating days, works well and that my abs respond better to nontraditional abwork (stability ball, planks, Pilates, Lotte Berk, etc).
I set fitness goals every birthday and since I turned 42 in December, my goal is to be able to do a teaser. (It helps for me to set specific goals in order to improve a problem area: teaser = stronger, more toned abs). Anyway, since my birthday, my rotations have included two days of Pilates (15-30 min), two days of Lotte Berk HHA (10-15 min), one day of stability ball work (PUB or QF) and one day traditional. I have noticed significant improvement in strength and definition. My skin may be too stretched out from my pregnancies to ever have truly flat abs, but when I hold them in (I'm GREAT at that!), they look darn good, if I do say so myself. They're actually not all that bad anymore when I don't hold them in. I still can't do a teaser but I'm getting closer!
Sorry this is so long, but hope it helps!
