“The qi (vital force) of yin and yang” refers to the two types of qi – an element that constitutes all things corporeal. With opposite attributes, the qi of yin and qi of yang interact with each other in accordance with certain laws. Such interaction is a catalyst for the generation and changing of all things. In addition, the circulation of the two types of qi may actualize mutual perception and influence between the things.

Between heaven and earth exist the qi of yin and the qi of yang. It is to man what water is to fish. The difference between qi and water lies in visibility. (Dong Zhongshu: Luxuriant Gems of The Spring and Autumn Annals)

Therefore, aware of the rising of wind, birds fly away before grass and trees start to sway. Aware of the coming of rain, fish swim to the surface to breathe before dark clouds start to gather. In both cases, the early reactions of birds and fish are caused by the interaction between the qi of yin and the qi of yang. (Huainanzi)