Since ancient times the Chinese have ascribed the source of all movement of matter and the life force to the constant, equilibrating pull of the negative and positive sides of energy, which they call the Yin (night) and Yang (day). This circle (or egg as others see it) is used to symbolize the origin of life. Yang signifies birth or day, Yin death or night. Because the two portions of the T'ai Chi balance each other perfectly, the symbol is also referred to as the 'Ultimate Principle of all matter.'
In Oriental art, medicine and philosophy, everything is classified under these two stems. To maintain harmony and order in the universe and within the body is to keep the Yin and Yang constantly in delicate balance. Chaos and disharmony abound when their even pull is disturbed in any way. Like opposite poles of a magnet, the Yin will attract the Yang but will repel another Yin force.
For the purposes of Chinese horoscope reading, it should be noted that the Yin or Yang only repels the similar negative or positive stem directly opposite its polarity. Only if the animal signs are directly across from each other is there any repelling action. Three positive or negative signs can be harmonious when they are 120 degrees away from each other.
Applying the Yin and Yang principle to the five elements in the horoscope may be a more complicated matter. For instance, the positive stem of Wood is a fir tree the negative is the flexible bamboo. The positive stem of Fire may be a forest fire the negative could be the flame of a lamp or candle that emits beneficial light. The positive of the Earth element, a hill; the negative, a valley. The positive of Metal, a gong or weapon; the negative a cooking pot or coin. The positive stem of Water, a wave; the negative, still water. The 'negative' here should not be taken in a pejorative sense as it does not mean something bad or undesirable in this context. Both the positive (Yang) and negative (Yin) forces have their good and bad aspects.
For instance, a person born under a positive sign will find him/herself more effective when he/she proceeds in active ways. Similarly, a person born under a negative sign is at his/her best when he/she behaves in a passive or non-aggressive manner. The first is compelled to be a doer and innovator, the second excels as a thinker and negotiator. Generally, Yang people are more spontaneous, Yin people more reflective and intuitive.
In the Chinese system of casting fortunes, the Yin and the Yang will appear alternately with the same element in combination with the twelve animal signs. Thus, the Yang (+) will be paired off with the Fire element first in, let us say, the year of the Rat (1936).
Immediately following, in the year of the Ox (1937), the Yin (-) force will have its turn to be combined with the same element, Fire, to balance off the Yang force.
We therefore find that there will be only six changes of elements in a normal twelve-year cycle, because each element must appear twice, once in its positive state and once in its negative state.
~from "Handbook for Chinese Horoscopes"
by Theodora Lau, 1979, Harper and Row.