These are my own observations. They may not hold for everyone. I am not an authority for anyone, except myself. But I thought to share these observations as they may be of benefit or interest to someone else.
In my opinion one of the main boons of proper Asana practice is a growing familiarity with switching more fluidly between the Para-Sympathetic and Sympathetic nervous Systems. Many forms of physical discipline are beneficially active, such as regular sports or gymnastics, and thus tone the Sympathetic Nervous system, which is good. Some physical disciplines are primarily slow and calming, such as Tai Chi, and thus tone the Para-Sympathetic Nervous system, which is also good. Asana, however, if practised correctly, offers an opportunity to become fluent in switching between these two branches of the nervous system. This can be very useful in ordinary life.
Some modern yoga practice emphasizes strong and demanding movement or stretching, maintained almost continuously throughout the session, relieved at the end by some rest period. This suits some constitutions, but does not develop proficiency in activating the Para-Sympathetic response.
The Sympathetic nervous system is concerned with activity, it generates the 'fight, fright, or flight' response, and is the portion of our nervous system most stimulated by modern lifestyles.
The Parasympathetic nervous system is concerned with the 'rest and digest' response, and can become neglected in a modern lifestyle.
When practising asana it is important to give as much respect, time and attention to the resting poses as to the active poses, throughout the session, thus giving the practitioner comfortable fluency in switching. Asana in its original intention, was the maintenance of a steady, still, relaxed pose.
When in a resting pose,
such as Shavasana- resting on back
Advasana or Jyestikasana- resting on front
Matsyakridasana - resting on side
Vajrasana or Shashankasana - resting in kneeling position or folded forwards
give attention to completely letting go of every unnecessary instance of physical tension in the body. Only those muscles and nerves required to maintain the posture should be engaged, and all other systems can release into inactivity and rest. Non-action is key.
Think leaden, molten, sinking, softening, loosening, untying, passivity, release, trust.
Be aware of areas of persistent tension that hold on even after you feel you have let go. The jaws, the teeth clenched, the fingers straight, the eye muscles tense, the forehead gathered, the anus, perineum and urethra sphincters tightened, the calf muscles ready to spring into action. Progressively let go deeper and deeper in all areas, and thus become accustomed to switching fluidly into Para-sympathetic mode.
There is nothing you have to do while resting. No anticipation of new movement. No memory of past effort. Nothing, absolutely nothing to do. For the whole duration of the rest, there is no anticipation.
Then when the time comes for movement, you will go towards it with freshness, as if it is the very first time you ever moved in that way, an innocent approach. And the sympathetic nervous response kicks in gracefully, and does its thing.
One more point, the neck and tail bone betray the nervous system. In an over-stimulated Sympathetic system the neck is slightly flexed back and the tail bone tilted out. If we get a fright we automatically experience this backward flex of the body. Severe trauma can lock in this response. This slight flex can also become so engrained from our reaction to ordinary life that our diaphragm (breathing muscle) remains somewhat tightened, thus shortening our breath. So when you assume a rest position be aware of making the smallest tilt forward to the neck and the smallest tilt forwards to the tail bone. It is a very slight tilt, a centimetre or so, very comfortable. Just to get out of the backwards flex.
Think of this cat to help visualise the difference.
Resting - curled
Scared - tail and neck back
That's about it.
For an indepth look at the Nervous System as pertaining to spiritual disciplines, those interested can check out Jana Dixon's awesome site