The fundamental concept behind Taijiquan and the basis for its martial applications is clearly represented by the yin-yang symbol for Taijiquan:
In Taijiquan, one almost never counters against an attacking force with an opposite frontal counter force with the purpose of overpowering that attacking force with a larger counter force. Instead, the Taijiquan practitioner usually sidesteps to avoid the frontal attack either completely, or partially and at the same time uses a small side force to deflect the attacking force. The Taijiquan practitioner basically allows the attacker to continue in the direction of his force, and utilizes that momentum to cause the opponent to lose his balance. Once the opponent senses that he may lose his balance and moves in the opposite direction, the Taijiquan practitioner also changes direction and counterattacks along the momentum of the opponent’s new motion.
The above illustrates at least two aspects of the yin-yang theory. One is the complementary aspect: complementing the attacker’s large frontal attack force (yang) by letting or even helping the attacker to continue to move in that direction with a small deflective and supporting force (yin). The other is the constantly changing aspect: constantly sensing the opponent’s movement and changing one’s counterattack appropriately. This is why in the above symbol, yin and yang together make up the whole, and yin and yang easily change into each other.
This above yin-yang concept is so fundamental to Taijiquan that it already manifests itself in only the second form, Wild Horse Shakes Its Mane, of the Simplified Yang Style 24 Form, which is often the first Taijiquan set that beginners learn. In the “Wild Horse Shakes Its Mane” form, the Taijiquan practitioner sidesteps an attacker’s frontal attack and grabs (or sandwiches the attacker’s attacking arm with the Taijiquan practitioner’s two arms) and “guides” the attacker along the direction of his momentum. This will cause the attacker to lose his balance unless he responds by changing direction and pulls back on his attack. When that happens, the Taijiquan practitioner then also changes direction and moves forward with his frontal foot and arm to cause the opponent to lose balance in his new direction of motion.
To increase one’s sensitivity to the opponent’s movements and intentions and therefore to be able to counterattack more effectively, the Taijiquan practitioner needs to keep his body, muscles, and mind relax, which is one of the fundamental principles of Taijiquan that is universal for all styles of Taijiquan. To facilitate the constant shifting of directions, one needs to keep the upper body upright, which is another fundamental principle of Taijiquan.
Because the movements in Taijiquan are relaxed and usually soft (although there is also a mixture of hard movements, especially in the Chen Style of Taijiquan), sometimes there is a misunderstanding that in Taijiquan one never uses force. This is a misinterpretation and is wrong. Even the famous saying “four ounces can deflect a 1,000 pounds” that is often used to explain Taijiquan does not say “zero ounce can deflect a 1,000 pounds.” This saying is trying to emphasize that one doesn’t need a counterforce of over 1,000 pounds to mitigate the attacking 1,000 pound force; however, to deflect the attacking force, one still needs to exert a small but non-zero force.
Once when I was teaching a Taijiquan class and explaining how one should maximize one’s force by synchronizing the movement of one’s legs, body, and hands simultaneously in the same direction, a spectator watching said “No, no, you are not supposed to exert any force in Taijiquan.” He was confusing softness with zero force. The need for a non-zero force is also clearly shown in the Push-Hands exercise of Taijiquan. If the opponent is attacking you with a Ward-Off, Press, or Push movement, you need to use some force to resist that attack, although the force doesn’t have to be large. If you use no force and retreat too rapidly, that will surely lead to a successful attack by your opponent. On the other hand, if you use too large a force to defend, you will also lose your flexibility and sensitivity allowing your opponent to counterattack successfully.
This fundamental yin-yang principle of Taijiquan requires the practitioner to be constantly sensing the opponent’s movement and intention, and complementing and changing his counterattack moves relative to the opponent’s moves. By utilizing strategic placements of his legs, arms, and body relative to those of the opponent, a Taijiquan practitioner can generate significantly more leverage. It is the application of the yin-yang principle, together with the principle that one’s power originates from the waist, that allows a smaller, weaker, older person to defend successfully against a bigger, stronger, and younger opponent.