They're at it again. "They" being the unscrupulous fitness marketers that tell tall tales to make their products sound better. The most recent one I've come across is for the Perfect Sit up, which by the way is designed for doing crunches, not sit ups. It comes from the same inventor of the Perfect Push Up(as if that exercise alone wasn't perfect enough!)

The commercial claims it will work your "lower abs." Not you'll feel it in the lower part of your abs but it will “activate the hard to reach, deep lower abs" as if you have a muscle in your body who's name is specifically "Lower Abs." The problem is you don't have lower abs any more than you have lower hamstrings or lower glutes.

The coveted, well publicized six-pack muscle group is the rectus abdominus. They're long, flat (I'm referring to the muscles themselves, not the entire belly!) muscles  that connect (originate) at the pubic symphysis (the joint at the front of the pelvis) and run vertically up the front of your trunk to connect (insert) at the xyphoid process (think CPR chest compressions reference point) and the cartilage of the 5th, 6th, and 7th ribs, and are separated VERTICALLY by the linea alba ( the up and down line when looking at a six-pack.) The three horizontal lines are created by tendinous inscriptions. Oh, and they're made visible by exercising and eating right, both in the correct amounts!
You don't have an upper set of abs that go from the bottom of the chest down to the belly button and another that goes from the navel down to your privates, each with their own separate function. That's what some infomercials would lead you to believe. The entire rectus abdominus has the main functions of helping to stabilize the spine and to flex the spine. Where the initiation of flexion occurs usually determines the region of greatest stimulation. In other words, the part of your body that starts moving your spine into flexion will determine the part of the abs that you feel working the hardest. Do a regular crunch and the abs will feel as if the area closest to the rib cage is working harder than the portion closest to the hips. Perform a reverse crunch and you'll probably notice the opposite. Do either version correctly and you will feel the entire rectus abdominus doing its job.

As far as the Perfect Sit Up gadget I have no opinion. The crunch, like the push up is already a great exercise by itself. I'm sure some people will find the contraption beneficial. But in my opinion if you're looking for home exercise equipment to enhance basic push ups or basic crunches I can recommend one piece of equipment that improves them both- a stability ball.