This is one of my own personal favorites, not easy to get into and out of but you really feel it working to increase the range of motion you have in the upper back (extension) and shoulders (flexion). Great for helping you finding comfort in yoga postures that require those sorts of openings such as Kapotasana, Urdhva Dhanurasana (Upward Facing Wheel) and Pincha Mayurasana (forearm balance).
Yoga is a state of being in which we experience ourselves as whole and present. To train ourselves in the state of yoga, we include the practice of physical movements for we are physical beings. We experience and express ourselves through a physical body, therefore, every so called spiritual practice must include the body in one way or another to be truly wholesome.
Back bending with ease and grace requires that the Hip flexors (front of the hips), shoulders, and the whole of the front of the body are flexible and strong. Yin yoga is a way of opening the body with passive stretches held for a prolonged period of time. If practiced correctly it is a very sure and safe way to work on increased flexibility.