This pose is a variation of parsva bakasana. It requires the same action in the spine, although the spine is often slightly more extended (towards neutral) in astavakrasana, which allows for a more even distribution of the rotation thought the spine.

In astavakrasana, the binding of the feet keeps the legs symmetrical. This symmetry in the legs and hip joints means that the rotation has to happen more in the spine and less in the hip joints. With the wrapping of the legs around the arm, less twist is needed than in parsva bakasana, because the bottom leg doesn’t have to reach quite as far. In parsva bakasana, the leg can release asymmetrically as the underneath leg slides forward, and the hip joints contribute to the rotation action of the spine.

As in ardha matsyendrasana, if the spine does not rotate, potentially risky compensatory twisting can occur through abduction or adduction of the scapulae on the rib cage.

Also, the wrapping of the legs around the arms creates a fairly stable pivot point. The challenge of this pose (if one can do parsva bakasana) is more about balance and flexibility than strength. The extended legs in this pose make the counterbalance on the support of the arms challenge.