Theory of Yin &Yang in Kung Fu

Yin Yang can be simply defined as two equal but opposite forces, each containing a little of the other, that come together to produce harmony. The Yin and Yang are a pair of complimentary and interdependent forces that act continuously in this universe.
In the symbol, the Yin and Yang are two interlocking parts of “one whole," each containing within its confines the qualities of the other.
The YIN can represent anything in the universe: negative-ness, passiveness, gentleness, femaleness, moon, darkness, emptiness or internal/softness. The YANG, the other half, stands for positive energy, activeness, firmness, maleness, sun, brightness, fullness or external/hardness.
The Yin-Yang co-exist as one inseparable force of an unceasing interplay of movement.
Firmness is concealed in softness and softness in firmness. Activity includes inactivity and inactivity includes activity.
Yang is the need to know of the conscious mind.
Yin is the need to comprehend of the unconscious mind.
Why then, is the Yin-Yang important to Kung Fu practitioners?
BALANCE or HARMONY...
We seek it in our training and in our lives.