The Yīn-Yáng 陰陽 Theory emerged in Chinese thought from the observation of nature. This theory postulates the existence of two complementary principles that are present in all that exists. These principles were called yīn, "dark", and yáng, "luminous"; they are not to be understood as two different natures but as two aspects of the same thing.
Origins and foundations:
The concepts of yīn and yáng are useful when describing the functioning of phenomena. To formulate them, the Chinese relied mainly on the observation of the relationship between light and shadow, usually explained using the example of the two sides of a hill: the sunny side, yáng, and the shadowed side, yīn. Visualizing this hill, we can understand that yīn and yáng are not opposing forces but two aspects of the same reality; they do not exist independently, but in relation to each other. Shadow exists because there is light, and vice versa. The ancients understood the universe as a manifestation of change, and everything that exists as a result of the interaction between these two polar opposites.
From this simile of the hill, opposed attributes were associated to each of these polarities: the yīn as dark, cold, feminine, soft, curved, rounded, Earth, moon, soft, wet; yáng as luminous, hot, masculine, hard, straight, angular, Heaven, sun, rough, dry...
In the ancient sexual rituals of rural China, held during the equinoxes, the men stood on the sunny side of the valley and called the women, who responded from the shadowed side. That is why it was said that "yáng calls and yīn responds".
The Yīn-Yáng theory also establishes certain laws in the relationship of these two principles:
1. All existing things have a yīn aspect and a yáng aspect. In all the existing we can find opposing aspects that we can classify as yīn or yáng.
2. Each aspect yīn or yáng can also be divided into yīn or yáng. The day, which is considered yáng in relation to night, can be divided into morning (yáng) and evening (yīn).
3. Yīn and yáng have the same origin and are mutually generated. Both are inseparable from each other and form the basis of their opposite. In a world without light (yáng), there would be nothing that we could call darkness (yīn), since the latter is defined in terms of the first: darkness is the lack of light.
4. Yīn and yáng mutually inhibit each other, are constantly changing and their increases and decreases are correlated. When one grows, the other decreases, and vice versa. During the day, as the sun rises, the light increases (yáng) and consequently the darkness (yīn) decreases, and vice versa. It is impossible for both poles to grow or decrease at the same time. On the other hand, this change is constant and does not stop at any time.
5. Yīn and yáng transform each other. When the sun is at its highest, yáng is at its maximum and yīn is at its minimum; from that moment the yáng begins to diminish and the yīn to grow, until the opposite point is reached, where the process will be reverted again.
From these laws, the existence of a balance between yīn and yáng emerges. When one increases, the other decreases, and vice versa, without there being an absolute predominance of one over the other. This is indeed the essence of life: change.
Yīn and yáng do not exist independently, but in relation to each other.
We must keep in mind that yīn and yáng are not intrinsic characteristics of things, but only concepts or labels that the Chinese used to explain the world around them. Through them the natural changes were explained: the alternation of day and night, health and illness, life and death. Therefore, the Chinese do not say that "this is yīn" or "this is yáng", but that "belongs" to yīn or yáng.
This theory was subsequently applied to Traditional Chinese Medicine (TCM), to classify body parts and physiological functions, and to explain the functioning of the human body in terms of health and disease. Health and illness are not to be understood as absolute realities but relative, and are governed by the same principles: they are constantly changing; when one grows, the other decreases, etc.
The Yīn-Yáng Theory in Martial Arts:
Martial arts is a discipline that uses the body as the main instrument. In the realm of the body, the Yīn-Yáng Theory is applied to the classification of the different parts of the body:
Front of the body
Internal part of the body
Back of the body
Upper part of the body
External part of the body
As a curiosity, we can mention a strike that in Choy Li Fut 蔡李佛 we know as Yum-Tsop 陰插. Yum is the Cantonese pronunciation of the word yīn, so Yum-Tsop is a blow that goes to the yīn part of the body, that is to say the lower abdomen or the groin.
Likewise, within the dynamics of movement, we can classify all corporal action following this same theory:
Leg without weight (empty)
Leg with weight (full)
Having this classification in mind can help us understand the dynamics of martial practice. The most notorious feature of Kung Fu is the use of any advantage that can be drawn on the opponent to achieve the highest efficiency with the lowest energy expenditure. For this, yīn and yáng have to work in balance.
The most classic example would be the way to deal with an attack. If before the hits (yáng) that approach always defend with hard blockages (also yáng), the result will be, if not an injury, yes at least the premature exhaustion. If we face the blows with yīn blocks, softer, that deflect the blow instead of hitting it directly, we will be making better use of our energies. Chinese Martial Arts are based on using the force of the opponent against himself, instead of opposing it. The absolute yīn in this case would be to leave the opponent's attack line so that his blow finds only the void and, if possible, hit or drag him in the same direction of his movement.
In the 'Snake Versus Crane' form (Seh Ying Duei Chuck Hok Ying Kuen 蛇形對拆鶴形拳) of Choy Li Fut, a two person combat form, we have a movement in which the crane pushes and the snake gives way completely instead of resisting the thrust, so that the crane rushes forward, only to receive the counterattack of the snake. This is an excellent move to show the dynamics of "yīn to oppose yáng".
Tài Jí Quán 太極拳 is a very effective martial art, based on this principle of not resisting and using softness against hardness.
The Tài Jí Tú 太極 圖 is a symbol that represents the interdependence of yīn and yáng, the generating principle of all things, called tài jí.
To give another example of application of the yīn-yáng theory to martial arts, I remember that our Sifu used to joke about a guard position in which there is too much tension. He said that if the adversary "felt like going around for a walk" before hitting us, by the time he did, we would be exhausted by the existing tension. This is a clear example of excess of yáng. The same thing would happen with an excess of yīn: if we are too relaxed we will not be able to defend ourselves. Relaxation and tension must follow the natural course of yīn and yáng: tense at the moment of striking, just at the moment when it is going to hit, and relax immediately after the blow, always alternating, seeking a harmonious balance.
In the same way, we could look for many other examples, but it is important to insist again that yīn and yáng are not absolute but relative qualities, and that they are only concepts that we can use to classify aspects of reality, as in this case we have classified actions and movements of the body, and not intrinsic characteristics. The essence of all this is the search for balance and naturalness, so that we can get the most out of our energy to be effective in the technique when we practice martial arts.