The reverse crunch is basically the opposite idea of the traditional crunch. The main muscle being worked is the lower abdominal muscles. The exercise can be done on the floor, or on a reverse crunch machine at the gym.
In the crunch the goal is to squeeze your abs in order to curl your shoulder blades off the ground. In this case were doing the same thing, except with our hips.
Your back should stay flat on the ground. Your hands can either lay at your side, or you can place your hands under your butt. As you curl your legs up towards your stomach, squeeze the muscles around your belly button and exhale. Your legs can be straight or bent.
Make sure you are raising your hips up off the ground or back pad, or else your hip flexors are really the only muscles that will be getting any work done.