Jing can be expressed by the hands, elbows, shoulders, hips, knees, legs, or even the body itself. Taijiquan emphasizes the upper limbs and the body, and uses the legs and feet as secondary weapons.
There are more than forty different kinds of jing excluding leg and foot jing. They can be distinguished into sensing jing and manifested jing. Sensing jing includes passive sensing jing, which are the ability to sense the opponent's power, and active sensing jing, which are the ability to move qi into or out of another person's body. Manifested jing visibly exhibit the use of force and can also be divided into yang jing or offensive jing (gong jing, 攻勁), yin jing or defensive jing (shou jing, 守勁), and neutral jing or neither yin nor yang jing (fei gong fei shou jing, 非攻非守勁).
Because offensive jing usually emit force onto the opponent's body, they are also called emitting jing (fa jing, 發勁). Defensive jing usually neutralize the opponent's power and thus are generally called neutralizing jing (hua jing, 化勁).
Offensive jing can be subdivided into pure offensive jing and attack with some defense jing (gong zhong dai shou jing, 攻中帶守勁). The first is strongly yang and can be represented by three solid lines. The second is also yang, but with some yin, and can be represented by two solid lines and one broken line.
Defensive jing can also be subdivided. First there are purely defensive jing, which are extremely yin and are characterized by three broken lines. Then there is defense with some attack jing (shou zhong dai gong jing, 守中帶攻勁). These are yin but have some yang and can be represented by two broken lines and one solid line. Almost every jing, both offensive and defensive, can be used as a hard or soft jing, long or short jing.
Before you start to study, you should know that many jing are mixtures of two or more other jing. For example, growing jing is a mixture of controlling, neutralizing, resisting, drilling, and coiling jing. Before you can understand a complex jing like this you must first learn the simpler ones. Second, you should understand that when most jing are used, they are usually coordinated with other jing, either to set up the opponent or to enhance the jing being applied. For example, in pushing hands, you must use yielding, neutralizing, and leading jing first, before you can apply controlling jing. Third, some of the jing are very similar to others, and you may find it difficult to differentiate between them. However, if you persevere in your practice and pondering, and continue to humbly seek the answer, you will eventually come to understand them.
Jing and Yi
Jing and yi (mind, will) are probably the most important keys to success in taiji. You should understand that if there is no yi, the qi will be stagnant, and if the qi is stagnant, then it cannot be applied to jing. Therefore, wherever the jing is to go, the yi must go first, and the qi will naturally follow. When you use your yi to lead qi in support of jing, the yi should always be farther than the jing; otherwise, the jing will be restrained and not completely expressed. This is best done by maintaining a sense of having an enemy in front of you. Imagination is the key here. If you only put your attention on your hands, the energy will have difficulty reaching them. When you put your mind on an imaginary opponent in front of you, the qi will more easily reach your hands and pass through them. Therefore, it is said: "Jing [can be] broken, mind not broken."
Jing, Muscles, and Qi
Almost all jing require using the muscles. Muscles play the major role in hard jing, a lesser role in soft-hard jing, and a relatively small role in the soft, pulsing jing used in taiji. As the role of the muscles decreases, the role of the tendons gradually increases, and the expression of jing changes from the slower muscular version to a fast pulse strike.
In Chinese martial arts, all of these jing require qi support. How does this qi support work? Let us digress for a moment and consider several factors affecting muscular strength. Most people know that we use only a portion of the capacity of our brains. The same situation prevails with our muscles, we use only about 40 percent of their capacity. There are several ways to increase this percentage. The will (yi) is important, sometimes to an astounding degree. When people are confronted with emergencies, they can sometimes do things far beyond their normal, everyday ability. When you decide you must do something, your body follows your will. Another factor that increases muscular strength is concentration. You have probably already observed that concentration increases the effectiveness of your exertion. When your entire mind is on what you are doing, and none of your attention is distracted by extraneous matters, all of your efforts and energy can be focused on one target.
Wherever your mind is, your qi is. Once you have developed grand circulation, you should be able to move your qi from your dan tian to any part of your body. When you put your mind (yi) on a particular technique, for example, a push, your qi will go to the muscles being used. The stronger your imagination is and the greater your concentration, the more the qi will flow and fill the active muscles. This energizes them the way a surge of electricity brings an electric motor to life. It will also increase the flexibility and elasticity of the muscles and make them feel inflated and capable of resisting a punch or generating a lot of power. The more disciplined your will and concentrations are, the more effectively you can move your qi.
Western science reports that concentration generates a chemical reaction in your muscles that increases the power much more than usual. I do not doubt that this is caused by the qi that this concentration of the mind brings.
When you have trained your qi and muscles, you will find that you can generate the same power with less muscle tissue. You will be able to relax your muscles, which will let the qi flow even more easily. The main bulk of the muscle fiber, which is in the middle of the muscle, will relax, and you will rely more on the ends of the muscles near the tendons. It is said that taiji jing comes from the tendons. This demands great skill and control on your part. To move in the loose, fluid, almost boneless fashion of taiji, you have to be able to place every part of your body in the correct relationship to each other part, the ground, and the target. The force generated by your legs is bounced off the floor and coiled through your body. The force cannot pass through any kinks or around any corners. Tension in any muscle will hinder the flow of force and qi. The tendons, which are the connectors of the parts of the body, transmit the force from each part to the next. The ends of the muscles use the minimum amount of effort to keep the flow going and direct it.
Most people have parts or patches of their bodies that they are not really aware of. This may show up as, for example, a place on your thigh where you are not as sensitive as elsewhere. This means that your spirit (shen) doesn't normally reach everywhere in your body, and consequently, your mind and qi don't go there either. You must train your spirit, mind, and qi to fill your body like a thread passing through every part of the pearl with the nine-curved passage. Only when your consciousness and qi suffuse your muscles can you really control them and your movement.
There are several ways you can fill your body with spirit and qi after you have completed grand circulation. You can visualize that you are a balloon being inflated with energy and awareness. However, the main method in taiji is doing the sequence slowly. When the form is done at a slow, steady pace, you have a chance to pay attention to the movement of qi without sacrificing anything else. Accumulate qi in your dan tian and coordinate it with your movements through your breathing.
You will always use some muscular energy (li) when you move. It is impossible to move without it. Similarly, there is always qi moving in your body. It only stops when you die, or a part of your body dies. An external stylist can develop qi to strengthen his movements, but it is local qi, developed in the arms and shoulders. Since this kind of movement uses tensed muscles, the qi from the dan tian cannot reach the arms. An external style of gongfu like Tiger Style uses this kind of local qi to greatly strengthen the hands and arms. A soft-hard style like White Crane uses this local qi to strengthen the arms, and balances it with attention to the waist and dan tian. Most of the time, however, the qi in the dan tian is not circulated up the back and into the arms. External styles exhale to get the air out and coordinate it with the energy, but it is stiff energy, not the soft energy of taiji.
(The above excerpt is from Tai Chi Chuan Martial Power Advanced Yang Style by Dr. Yang, Jwing-Ming.)