I'll give this a go since abs are a subject near and dear to my heart. I had back surgery a few years ago and worked with my PT to get my abs back as they are even more important now.
In my opinion (I have a personal trainer's certificate from ACE, too, so I have a little education) most reverse crunches are done too fast to really benefit from them because most folks hoist their hips up using too much hip flexor and momentum, leaving the abs a lesser role in the party.
What I like to do is slow down the reps and include a brief hold at the top of each rep to really isolate the abs. I also think using straight(ish) legs helps with the idea that you need to lift the legs up and not draw them toward the chest. Drawing them toward the chest isn't a waist of time, it just doesn't tax the abs as much because you're fighting gravity less.
When you first try this it may be frustrating because you may feel like you are only raising your tailbone 1 millimeter off the floor. But be patient and you will get stronger quickly.
I love Cathe's ab routines partly because they are long and grueling, but that fact may make it hard for you to work on form for this particular move, so I suggest you set aside a few moments after your regular workout to bang out a few reps of reverses with total concentration.
Also, bear in mind that the lower abs are a part of the whole six-pack muscle known as the rectus abdominus, and that strictly speaking the whole muscle contracts every time any of it is worked so different people may have different levels of success in trying to isolate one end of the muscle. (Are you top-heavy or bottom-heavy; are you leaving your head down while doing reverse crunches or keeping it up, etc.)
Another thing to be careful about is when doing variations involving lowering the legs toward the floor between reps, do not go so far that your lower back pops off the floor; that strains the disks in the lumbar spine.
Hope this was helpful.
--Ann
