Keep your legs straight and lie down. Place the hands by the sides of the body. Now relax the nerves in the feet. Then gradually relax the upper limb nerves. Just as a dead person has no dominion over the nervous system of the body, i.e. free the nervous system of the entire body from its domination, i.e. relax all the nervous systems and remain dead. Keep the mind completely calm and neutral. Make the mind free of thoughts just as there is no thought in the mind during sleep. In this way, by making the mind free of thoughts and relaxing the body, lying dead for 15/20 minutes removes all the fatigue caused by asana and posture habits. This asana should be done last after the asana, mudra and pranayama rituals. It is both physical and mental relaxation.

Uttan sabbabhumou shayan sabasanam.

Shabasanang Shramaharang Chittashrantikarak..

(Gherand-Samhita-2/17)

The body of the practitioner of this asana apparently becomes dead and inert, hence its name Shavasana or Mritasana.