In this challenging pose, Eka Pada Rajakapotasana (One-Legged King Pigeon Pose), the chest is puffed up to resemble a king pigeon. And while we certainly don't walk around strutting our stuff, this asana will energize your adrenal glands and fully open the front of the body.
It is challenging for most people to make the thoracic spine (upper back) flexible. The ribcage attaches to the upper back, which limits its mobility. And the shape of it is naturally convex, which is why we often see the chest collapse in sedentary people. The spine, however, can become supple. And while it is obviously quite difficult, we should distribute the curve evenly throughout the length of the spine in all backbends. Ever see a palm tree in the wind? The curve is long and smooth, starting at the base.