Yin and Yang are polar opposites. If we think in terms of circuitry Yin is negative and Yang is positive. Why is this important in Kyusho? When you touch positive and negative on a car battery you get a spark. The same principle applies with the human body.
Designations of Yin or Yang are made only in relation to each other. There are no absolutes. For example, the sun is bigger, brighter and hotter (Yang) in relation to the earth which is smaller, darker and cooler (Yin). However, the earth is more Yang in relation to the moon.
All meridians are classified as either Yin or Yang. The front of the body is yin, the back is yang. The left is yin, the right is yang. The top is yin, the bottom is yang.