The key to understanding YIN and YANG is to realise that there is always an underlying third member that is the silent partner. This third member represents balance and harmony.
In our body we must balance the Yang CHI received from our father's sperm cell with the Yin CHI received from our mother's egg. The point to remember is that our Original CHI was created from equal doses of Yin and Yang. In Chi Kung, Yin and Yang manifests itself as active and receptive. Any two thoughts, ideas or actions that are opposed to each other or balance each other, form a Yin and Yang relationship.
Yin and Yang seek balance. In our bodies this balance is often lost. As the strength of the original CHI weakens, it loses the ability to keep us in perfect balance.
Yin and Yang can be thought as two poles of a battery. One is negative and the other is positive. Together, they work to keep the body and it's component parts in balance. If one pole goes out of balance, it must be adjusted by an infusion of energy from the opposite pole. Our upper body, above the novel is considered Yang and the lower body is considered as Yin.