This counterposes the shoulder action of salamba sarvangasana & niralamba sarvangasana because the signal extension and scapular adduction of should stand are reversed, so the muscles that were active are now lengthening. If the release is too passive, however, the muscles can be overstretched.
The support shifts in this pose to the spinous process of the thoracic spine from the scapulae and trapezius.