Yin/Yang
This is the real Tai Chi! This is the base of all movements in Wu Gong. Qi flows from the constant changing of yin/yang.
‘Yin/yang’ is the rule of opposites, balance, flow and change. Yin/Yang is the basis of everything in the universe. If one were to start from the very beginning of existence (yang) and non-existence (yin), one can see everything has an opposite to create a harmonious balance. Male (yang) and female (yin) are as one in the procreation of life. Up (yang) and down (yin) is the relationship from where one is looking. Full (yang) and empty (yin) make things possible to be what they are: a cup is only a cup from the emptiness within a cup. Light (yang) and dark (yin), as with the sunshine (yang) and shade (yin), also more obvious is day (yang) and night (yin). Although good and bad/evil are often seen as opposites in the West, they are not included in the philosophy of yin/yang. Good and evil are abstract values, a moral judgment. Yin/yang are the balance of a concrete nature that forever changes for life to flow.
The use of Yin/Yang is in everything we have, do, see, think etc... It is used extensively in martial arts and in Traditional Chinese Medicine. It is the 0's and 1's of the computer world but are represented as
for yang (aka a solid line) and 
for yin (aka a broken line). These are used in the Ba Gua and the I Ching to represent the Yin/Yang. If you want to have good health, wisdom, and happiness, study the Yin/Yang balance and practice following the nature of the Dao.
What are the characteristics of the yin/yang relationship?
Yin/yang training is a way of seeing the world.