Azmi has added the backbend poses for the Ashtanga Led 2 classes and Kapotasana is one of the poses. Whenever I do the pose, I would feel compression in my lower back even though I am just lifting my hips off the floor with my head resting on the floor and my hands behind my ears. Azmi has advised me to wait the pose out as my body is not ready for it as my shoulder is not opened enough for me to do the pose. He said that the pose will come to me and it would be worth the wait like it did for Urdhva Dhanurasana (Wheel pose). Meanwhile, I would need to get my shoulder to open up more and the following pose is recommended by Azmi.
Bend your elbows 90 degrees, and carefully place the backs of your elbow on the front edge of the chair seat, about shoulder-width apart or slightly closer and clasped your hands together. Walk your knees away from the the chair until your trunk is parallel to the floor and your knees are directly under your hip joints. Draw your front lower rib cage upward so it does not sag toward the floor, and keep it there throughout the pose. Exhale and move your hips horizontally backward to lengthen your spine, slide your outer shoulders toward your ears, and draw your head away from the edge of the chair seat. Allow your head to hang down in the space between your trunk and the chair. Keep your hand pressed together and squeeze your elbows towards each other to find a strong and yet comfortable stretch.
This article "Shoulder Responsibility" in Yoga Journal writes about shoulder movements require for the backbend poses which is an important aspect to prepare the shoulder opened for the poses.