In keeping with his emphasis on intensity, Mr. Iyengar used to conduct what he called “intensives” for his pupils. In 1991, for example, he taught an intensive on backbends for his senior teachers.
One of the advanced backbends poses is Eka Pada Rajakapotasana I (One-Legged King Pigeon Pose).
Part of the difficulty and complexity of the pose lies in the positioning of the legs. One hip is in an extended position, as is typical in backbending poses. The other hip, however, is in a flexed and externally rotated position, which is unusual in backbends. This creates difficulty in balancing and aligning the pelvis, and as a result, makes moving evenly in the spine, especially the sacrum, quite challenging.
Another challenge in Rajakapotasana I is the opening of the chest and shoulders that enables you to reach overhead and hold your foot without collapsing and compressing your lumbar spine.
In order to ready yourself for both of these challenges, here are a couple of preparatory exercises. Prior to practicing these preparations, build up some heat in the body and open the shoulders and hips with standing poses, Adho Mukha Svanasana, Adho Mukha Vrksasana, and Pincha Mayurasana.
Read more about this very advanced pose on http://www.yogajournal.com/practice/176