twiiq:

Yin and Yang are dependent opposing forces that flow in a natural cycle, always seeking balance. Though they are opposing, they are not in opposition to one another.

As part of the Tao, they are merely two aspects of a single reality. Each contains the seed of the other, which is why we see a black spot of Yin in the white Yang and vice versa.

They do not merely replace each other but actually become each other through the constant flow of the universe.