The tao represents the concept that everything, no matter how detailed the refinement, contains both sides of a duality in some amount. If you can define it then there is a comparison. A comparison is based on what something is as well as what it isn't. A definition also requires that you seperate the duality from the rest of existence. All duality definitions contain the interaction between the two as a result of the process. For example to breath in or breath out requires that you define a full exhale and inhale. however all the attention is focused on the breath which is an interaction of two definitions. The interaction is where the power comes from. All meditations and all martial arts use the concept. This includes yoga, boxing, and all other arts. Just because it isn't asian doesn't mean it is not an art.
When you (and/or the person behind you) read about this in books, they usually have a list of things that represent either yin or yang. This list is simply for comparison. Anything can be yin and anything else can be yang depending on your definition. In application Yin tends to be receptive and Yang tends to be active. For example Night can be defined as causing fear and therefore yang while fire brings comfort by absorbing the darkness and is yin. The definitions do not need to have a basis in reality. They only have to make enough sense to be used and understood by you, the person using them. You may notice that most kung fu explanations don't seem to make sense. Nobody really cares because the training is the most important factor. The training brings experience. The experience eventually bottoms out and the returns diminish. The tao is taught at this point (or at least yin and yang are explained) and all experiences are re-examined under this concept. Definitions that hinder training are examined and changed until they help. At this point people begin to develop unique theories that help THEM train. This is why masters will show you what to do rather than explain a concept. They know that you won't understand them but they know it works. The theory doesn't matter as long as the movements are practiced. With practice theories develop anyway and why should they get in the way of that.
Experience is the only thing that can be duplicated through training anyway and theory just gets in the way. Many times the training will give a theory that is shared by people, this becomes a style. Each style has a unique way of defining yin and yang and many times each animal style will further diverge from that definition until the advanced levels of a style begin to be unrecognizable. Keep this in mind when reading since all definitions are a combination of My research and MY training. The information may be totally diferent from what your experience and research has taught you. That is okay. If we had the same experience then we would have the same training and this web page would be a waste of time and effort.
Good Luck and Train well..