Xin and Yi are the Dao in Taijiquan. Lao Zi, Chapter 36 said: ‘Wish to close it, must first open it; wish to weaken it, must first strengthen it; wish to abolish it, must first raise it; wish to take it, must first offer it.’ It is also said in Taijiquan classic: ‘withdrawing is releasing and releasing is withdrawing.’ It means: ‘Yin is Yang and Yang is Yin.’ Though they are opposite, they are the two faces of the same object which cannot be separated from each other or exist alone. If it can exist alone, then it is a Wuji state and not the natural rule of Yin and Yang.
Yin and Yang are opposite to each other but not absolute. How Yin or Yang are defined depends on the perspective of the viewer. Often, what is considered Yang by one person can be Yin to another. Moreover, Yin and Yang are exchangeable and this exchange also depends on how you change your viewpoint. You should also remember that the mind is what is called Taiji or Dao in Taijiquan. When Yin and Yang concepts are applied in Taijiquan, Yin can be Yang and Yang can be Yin. Yin can also be Yin and Yang can also be Yang. All of these alternative concepts all depend on your mind and consequently, different strategies are derived. That means an insubstantial action can be a setup for a substantial action and vice versa. If you do not have the options of Yin and Yang exchange, then your strategy will be stagnant and be easily defeated by your opponent.
Generally speaking, Taijiquan’s Yin and Yang can be: closing and opening; defensive and offensive; bending and extending; inhaling and exhaling; retreating and advancing; insubstantial and substantial; small and big; neutralizing and emitting; refined and coarse; leading and attacking; soft and hard; internal and external; Yi and Xin; raising and falling; looking up and looking down; coming and going; enter and exit; withdrawing and releasing; etc. However, (you) should not be fixed in the rules without knowing the mutual exchangeable theory of Yin and Yang. For example, during combat between the opponent and me, bending can be Yang as offensive, and extending can be Yin as defensive. Raising can be Yang as a substantial action and falling can be Yin as an insubstantial movement. Coming can be Yang as emitting and going can be Yin as storing.
This paragraph lists many examples of the Yin and Yang aspects of Taijiquan actions. However, you should always remember that Yin and Yang are mutually exchangeable. That means substantial can be insubstantial and insubstantial can be substantial. If (you) are able to understand the theory of Yin and Yang and knows their applications, then (your) comprehension is deep and (Taiji) knowledge is profound. Insubstantial and substantial are exchangeable and are mutually Yin and Yang. This will make the opponent lose track of catching the (Jin’s) coming and going. Then this is the beginning of understanding Jin. The theory of Yin and Yang is hard to change; however, the applications of Yin and Yang are exchangeable. Therefore, (Taijiquan) practitioners should be always researching and pondering the theory of Yin and Yang and searching for the applications of Yin and Yang’s mutual exchanges. If (you) are able to catch this knack and apply it skillfully, then (you are) surely a proficient Taijiquan talent.
You should know that Taiji’s Yin-Yang theory is the foundation and the root of the entire Taijiquan’s creation and development, you should always ponder the theory and its applications. Theory is the scholarship of the art while the actions are the applications of the theory. Only then can you catch the crucial key to Taijiquan’s substantial and insubstantial. Once you are able to apply this Yin-Yang theory into the Taijiquan martial art, you will be able to know the opponent but the opponent will not know you. When you have reached this stage, you have surely understood the applications of Understanding Jin.