Four Properties of Yin & Yang
There are four basic properties that help to understand the relationship of yin and yang and make it easier to apply these prinicples to the microcosm that is the human body.
1) The Opposition of Yin & Yang
Yin and Yang are oppossite, however only relative to each other. Nothing is wholly yin or wholly yang. Each contains even the smallest of seeds of the other inside it. At any given time the two are in a constantly changing balance, with each vying for that one step ahead. Yang natured things (e.g., heat) counter and dispel yin natured things (e.g., cold) and vice-versa. If one predominates, it can overact on the other, cause imbalance and lead to disease.
2) The Interdependence of Yin & Yang
Yin and yang, though relatively opposite to one another, can not exist independently. They define each other, much as night and day do. One cannot know light without dark, or dark without light. They only exist in relation to the other. In addition, they feed off of each other. Yang is energy, and it needs nutrients to exert itself. Yin is nutrient and needs energy to form.
3) The Mutual Consuming & Supporting Nature of Yin & Yang
Being in a constant balance, yin and yang are constantly attempting to adjust to levels of the other. Outside influences may cause levels of one to either become significantly greater or lesser than the other. Four possibilities can occur: Yang Deficiency or Heat Xu, Yang Excess of Heat Shi, Yin Deficiency or Cold Xu, Yin Excess or Cold Shi.
There are two things to take note of here. The first is to pay attention to the heat and the cold attributes. Interchanging the words yang and yin with heat and cold respectively, plus combining with deficiency (xu) and excess (shi), you can get a rather good sense how to apply this to the body imbalances. For example, yin deficiency can also be thought of as cold deficiency. With a lack of cold in the body, false heat signs can be seen. The second thing to note is the level of the non-deficient part of a xu/deficient condition. Since yin & yang draw off each other to exist, any deficient condition will cause a general deficiency of both yin and yang. For example, yin deficiency will also have a minor deficiency of yang that puts it just below its balanced level.
4) The Intertransformation of Yin & Yang
The dynamic balance of yin & yang is such that the two can transform into each other. Summer will eventually turn to winter, day will become night, even the most wild sugar rush will eventually lead to a resounding depression. The change is not spontaneous but needs certain factors or precursors to exist. Internal factors are primary, yet external factors also have an effect. In addition, the timing of these things must be right. Consider the transformation of matter to energy. The internal conditions of the composition of matter must be right and external factors must be applied at the right time for Mass to multiply by C2 and cross over that equal sign to Energy (wonder where Einstein really got the notion of E=MC2 ?).