Qi divides into 2 units of existance - that of Yin and Yang. These dual entities oppose, compliment and consume each other. The interplay of YinYang creates all things.
Yin is the feminine principal, the dark, the substance of things, whereas Yang is the masculine, the light, the energy of things. One cannot exist without the other, like two sides of a coin.
Within Yin, there is always Yang
Within Yang, there is always Yin
The continuous transformation is never absolute in that a seed of the opposite is always present within the other. Hence the well-known symbol of Taiji has a small dot of the contrary colour within each of the fish shapes.