In the Iyengar system, this pose is practiced sitting on the foot. Not just any way of sitting on the foot. You must sit on the edge of the foot with the bottom of the foot facing the wall behind you. This is uncomfortable for almost everyone on the planet, but is especially uncomfortable if you have stiff ankles or have recently hurt your ankle.
I have recently hurt my ankle. I wish to tell my readers that I hurt my ankle snowboarding, doing amazing tricks on slack ropes, or breaking someone out of prison, but alas, it was walking on a sidewalk in my neighborhood. It just twisted and stopped working the way ankles are supposed to and Ardha Matysendrasana suddenly became even more difficult.
Props to the rescue. Here's the set up:
This is the Junior 1 and Junior 3 version. I'm holding the blanket rather than my foot. This is another advantage to having the blanket.