Who does these? And who knows anything about them?
Here is my knowledge from reading bullet proof abs by pavel:
These are awesome, basic idea is that when you flex your hamstrings you cause your hip flexors and other muscles to relax so you really hit the transverse abdominals which are the six pack muscles.
I have been doing these a little bit in my basement using a band set up. Pavel sells a simple piece of equipment but here is my set up. I put a mini band about half way up a pole in my basement and then put my legs through it. I then force my legs down by flexing my hammies and getting my feet flat on the floor. While keeping this tension, I then do a sit up.
These are incredibly hard and if you have never done them you will probably will not be able to do one. So I also place a band higher up on the pole and use it for assistance. These look and sound simple, but they will crush you.