Sometimes the obvious is the hardest to see. I think that applies to Yin & Yang, a symbol the is ubiquitous in martial arts, healing & energy work.
We talk about it, put it on our t-shirts, hang it on the wall, its beady little eyes staring at us all the time with its ancient esoteric wisdom that we so clearly understand. This side is yin and this one yang. This leg is empty and this one is full. Hard vs. soft, tense and loose, up and down, inside and outside, etc. etc. Yin And Yang seems pretty straight foreword, next lesson.
In actual fact I think we look past it or even through the YY idea without thinking much at all about renewing, exploring and evolving our understanding of it. If relegated to an arm chair philosophy, the theory of Yin & Yang can languish as sad and unappreciated as that Samurai sword collecting dust on the wall, it can be a sharp edge that cuts nothing.
In working with people from a variety of martial arts and healing modalities I often observe a temporal mistake in the application of YY. What folks are often doing in their practice is applying yin then yang (or visa versa) instead of yin entwined with yang.
This is a very easy mistake to make. We tend to think temporally in most things....I put this down to pick this up, socks go on before shoes, learn to walk before you run, that sort of thing. Typically we don't think of doing two things at once or, in our case, two seemingly opposite things coexisting in a mutually supporting way.
In my experience the YY model is attempting to communicate how things unfold in real-time. Its an observation that the extremes are mutually supportive and, in fact, aspects of a larger whole. It suggests that the ultimate extreme is not an end, but a beginning and traces a middle path of balance between the seemingly opposed forces. I like to think of it as an infinity symbol folded over on itself.
What does that mean practically?
One of the most common problems involves the vertical axis where the intentional projection upward is lost when sinking down and/or the downward sinking is lost when projecting upward. Sinking down and stretching upward must both be present to engage the vertical linkage. Even more importantly they are not 2 different things but two aspects of one thing. Unity or oneness is the goal.
The old saying is stretch the tendons and contract the bones, not stretch the tendons then contract the bones.
I have often heard Grandmaster Wang referred to as 'Old Man Contradiction' for his tendency to point out seemingly opposite things that actually work together. You could say our conscious reality is the ceaseless interpolation of opposites within a unity that always remains balanced in itself. Or you could just notice that our bodies also work according to this model (the model merely being an conceptualized observation) and use it to figure out how to punch much, much harder.
Sometime I find it useful to just switch my emphasis. Let's say I have been working on relaxation or looseness within my frame. Why not switch to focusing on the role of tension for a bit. If I've been keen on rooting for a while, lets emphasize branching. If I have been practicing at a very slow speed, why not try fast?
Looking at it this way you can kind of see Wang's thinking in designing Yi Chuan. Because there are just so many aspects of our being that we can work on at ANY given time why make it more complicated than it needs to be?
Why not use big, simple shapes and routes so that we can study what unites them all.