The Asanas are based on the principle that you cannot relax the body without tensing it in some form. For instance, to relax the muscles of your thigh, the best way is to first tighten the muscles. Hold it tight for a while. And, then let go. And, you see, how relaxed they become. So, all the Asanas are based on first tightening the muscles and the muscle fibers, including the spinal cord, and then to relax them. Let go, so that they are completely relaxed.