Here are the differences between these two poses, which intriguingly enough, are often taught as one is many classes. Shoulderstand (Sarvangasana ) and Psychic union pose (Viparita Karani mudrasana)
Shoulderstand / SARVANGASANA
* More advanced -- it is the later stage of viparita karani mudra. You have to transit the VKM first to reach this.
* Tremendous pressure at the throat, deliberately induced to create the jalandhara bandha or chin lock. This lock prevent the leakage of nervous energy upwards.
* Difficult to hold for long, unless you practice it regularly.
* Cannot be done with a prop.
* Increases pressure on thyroid and is used to harmonize this gland and correct imbalance of hypothyroidism
* For same reason, helps lose weight naturally.
* Stresses the neck in a positive way, but must not be attempted while having cervical problems.
* The neck and shoulders take the brunt of the pose.
* Contraindicated in certain problems, including blood pressure or cardiac issues.
Psychic union pose/VIPARITAKARANI MUDRASANA
* Could be a preparatory pose for shoulderstand, though it is a complete pose in itself. Mastery in it is essential to reach sarvangassana
*: Less pressure at the throat. Is easier on the neck, and more pressure at the lower back
* Duration in it may be progressively increased to great lengths, even for those not very adept at other inversions. To even five minutes or more.
* Can be done with props -- a wall, bolster, chair etc..
* Has a mild impact on the thyroid, but not as intense as sarvagasana.
* Helps weight loss, but not as intensely as sarvangasana. A lot of heavy waisted women hold this pose comfortably, and may not effectively use it as a weight loss pose if held too low.
* The stress at the neck is minimal, if you learn to hold the pose properly, with the legs low, and feet over the head.
* Upper back takes the brunt of the pose.
* May even be taught to heart patients after regular yoga practice of other poses. And may be done with prop, and actually help the heart heal. Is sometimes called the cardiac pose for this reason.