I'm not going to explain this exercise because everyone has done it before and there are a million places and people (at the gym, on the internet, on DVD's, your grandmother) that will tell you how to do this exercise.
My advice to you; if you don't feel it in your abs, then you are doing it wrong. There is a lot of debate on where you put your hands; some say across your chest, behind your head, out in front of you. It's a crunch. It's really not that big of a deal.
