"Taiji Jin includes: (1) Nei Jin (Yin) , and (2) Wai Jin (Yang); also (1) Defensive (Yin), and (2) Offensive (Yang)
"There are two ways to classify the Yin and Yang of Taiji’s Jin (power). The first way is according to how the Jin was generated, and the second way is according to the purpose of the Jin.
"First, Taiji Jin can be classified as Nei Jin (Internal Jin), which is Yin; and Wai Jin (External Jin), which is Yang. Nei Jin training is the critical key which enables the Wai Jin to manifest its maximum power. Nei Jin includes how to build up the Qi to a higher level and how to lead Qi from the Lower Dan Tian to the limbs to energize the muscles. The mind is extremely important in Nei Jin training, and the methods are critical. The master usually does not reveal Nei Jin training to the student until he can be trusted.
"Wai Jin concerns the physical movements of the Jin, which include the movement from the root of the stance, how to use the waist to direct the Jin from the legs to the limbs, and how to manifest and use the power. To use the analogy of a machine, Wai Jin is involved with how strongly the machine is built, while Nei Dan is concerned with the amount of energy which is put into the machine. Remember: the power and efficiency of the machine is determined by the energy supply.
"Next, if we classify Jin according to its purpose, then the defensive Jin is Yin while the offensive Jin is Yang. Typical examples of defensive Jin are Listening Jin, Yielding Jin, Leading Jin, and Neutralizing Jin. Typical examples of offensive Jin are Pushing Jin, Striking Jin, Drilling Jin, etc.
"Jin can also be classified according to how the Jin is manifested. For example, Jin which relies more on muscles than Qi is classified as Hard Jin (Yang), while Jin which reduces the use of muscle to a very low level is classified as Soft Jin (Yin). Naturally, like all other cases of Yin and Yang classification, these can be further sub-classified." *
* Dr. Yang, Jwing-Ming, The Essence of Taiji Qigong