The shoulderstand is the second asana we practice in the series, following the headstand. As an inversion, it allows the normal flow of circulation to be inverted, bringing oxygenated blood to the torso and preventing blood from stagnating in the legs. It also strengthens the back muscles and massages the shoulder blades.
Swami Vishnudevananda (bringer of yoga to the West) on shoulderstand:
This exercise is similar to headstand. In headstand, circulation and concentration are directed to the brain, but in shoulderstand the concentration and circulation are directed to the thyroid and parathyroid. The thyroid is the most important gland of the endocrine system, and this exercise gives it a rich supply of blood.
Fifteen minutes is the maximum for this pose; starting time for beginners is one minute.
The Sivananda Companion to Yoga says of sarvangasana:
The asanas in this cycle are mentally far more approachable than those in Headstand Cycle, for the simple reason that with your head face up on the floor, you can see what you are doing. This not only reduces your fear of pitching your body into new positions, but it also means that you can act as your own teacher, checking that your body is straight or your limbs are symmetrical. The cycle concentrates prana [energy] on the neck and upper spine area and this in turn brings great benefit to the lower back – since any work done at one end of the spine is automatically reflected in the other end.
The shoulderstand is followed by the plough.