This pose can be done while focusing on using spinal extensors (which include the psoas major on the front of the spine) or supported on the elbows. If the support of the elbows is used, there is less work in the muscles of the torso and perhaps more ease in breathing and more expansion.
If the pose is done while focusing on the muscles that extend the spine, the neck will be better protected when lifting the arms of the floor. Variations can also be done with blocks under the spine, and with the feet in baddha konasana or padmasana.
This pose is a great demonstration of the role of the psoas major in both hip flexion and spinal extension.
Fish pose is frequently used as an immediate counterpose to the shoulder stand because it reverses the position of the cervical spine from extreme flexion to extreme extension. However, going from one static extreme to the other may not be the most beneficial way to compensate for the stresses of the should stand. A more therapeutic approach would be to gradually reverse the movement of the neck with simple vinyasas leading up to the cobra pose.