'Yin' yoga, as one might guess, is the other part of 'yang yoga'. It involves getting into more passive poses and holding the positions longer. Many of the poses require a wall to lean on. I'm all for walls.
Leaning is good too. Things would fail and eventially fall down if they didn't lean on each other. The stones on both sides of a dry stone wall are usually placed so that they lean into and touch each other and so counterbalance each other and thereby maximize their stability.
We can all learn things from walls too, especially stone ones. Like how to be still for long periods of time. How to be quiet and learn to wait. Leaning up against a tight wall we can meditate on what 'fitness' really is. My guess is that the stones in a well built wall are probably much better in touch with each other than we are with our own bodies.
They say a yoga pose like this one with the legs straight up against the wall is good for loosening tense muscles, helping with back problems and aches & pains from over working the body. I'm in a position to find out .