With crunches the back remains supported, either on the floor, or with an exercise ball like the Bender Ball as opposed to sit-ups where the back is actually lifted up off the floor.
Begin crunches by lying down on the floor on your back, with knees bent. You begin the crunch by curling in the shoulders toward the pelvis. Hands are kept behind the neck, or else crossed over the chest. Since pressing with force on the head or neck can cause injury it is a good idea to take care to not place the hands behind the head itself.
It is actually useful to create a type of shelf to rest the head in with your hands, to support the weight of the head so the neck flexor muscles are in a relaxed state during the exercise. There is no fear of injury to the neck as long as the neck is kept extended and the neck flexors remain relaxed, since the hands will not be putting extra force on the head.
Crunches are not only safer than sit-ups, but they work better to isolate the abdominal muscles for exercise, getting more effect from less effort. Since the hip flexors are used during sit-ups, some of the energy expended is used to contract the hips and the hip flexors in turn take away some of the movement from the abdominals.
