The crunch is an abdominal exercise. Here you lie on your back, the legs are angled.The hands are either loosely behind the head or in front of the chest. The view will follow above. Now lift the upper back so far that the shoulders remove from the ground. Then you lower again, but the back does not touch completely the floor. It remains a slight tension. The lower back remains completely on the floor. 

https://www.youtube.com/watch?v=Xyd_fa5zoEU