Iyengar Yoga provides many great alternate poses to help teach us the mechanics of a pose. One of these great alternates is chair shoulder stand. In chair shoulder stand, the practitioner is inverted with their shoulders on a bolster and arms clasping the chair rungs at the back of the chair. The tailbone is resting on the chair seat with either the legs resting against the wall or raised toward the sky.
One question I have received in the past when practicing chair shoulder stand is, "why not just get into the traditional shoulder stand?" The reason is that this alternate version of the pose provides us some education that we cannot get from coming into the full pose. In traditional shoulder stand, there are many variables to think about: coming up high on the shoulders, supporting the back and neck, moving the tailbone in and legs back, rolling the legs towards one another, flexing the toes, etc. The chair helps us to get into the pose in a supported way, so that some of these variables are instead put onto the props.
Being able to really access the tops of the shoulders is a blessing that this pose provides. By having the tailbone supported, and the chair rungs, the student can continue to turn their arms out, bringing the shoulder blades in, and are able to come onto the tops of their shoulders. For many new students, it is hard to access this without the use of props. More often than not, new students are actually standing on their upper back, not their shoulders. By utilizing these props, students can not only access the pose, but also get a greater understanding of where the base of the pose is.