In the courtyard of his home, Chen Xiaoxing corrects a posture
Tai Chi seems to have so many confusing requirements. However once you meet these requirements there is a very specific feeling of fullness you get in your dantian. This feeling is probably the best indication that what you are doing with the rest of your body is correct. When you can remember this feeling and produce it by adjusting your body, then life becomes a lot simpler. You can stop thinking about the multitude of specific requirements and just think of this one.
Some of the commonly talked about body requirements are: head suspended, shoulders relaxed, elbows sunk, chest held in, back relaxed, dantian full, kua relaxed, strength above the knees. These requirements apply all the time, but apply most strictly when in a static posture. What they do is allow the bodies qi to trickle back to the dantian. No matter what posture you are in, a simple upright standing meditation or a low 'dragon on the ground', if you follow these requirements you qi will centre in you dantian.
Tai Chi has a lot of body skills. In general, the body can be divided into three sections: the hips, the shoulders, and the dantian. The hips include the pelvis, the hip joints, and the lower most part of the spine. The shoulders include the shoulder blades, the shoulder joints, and I believe for all intents and purposes the rib cage and upper spine.
In many respects the hips and the shoulders follow the same rules as each other. Its just their physical position that differentiates them. The shoulder blades and the rib cage are analogous to the pelvis, only they are a lot more flexible and articulated. Likewise the arms and the legs are really the same. Sinking the elbow means that in an open posture the triceps are engaged in opening your arm. This is analogous to the feeling in your legs when you engage your quads (the thigh muscles on the front of your leg) to support your weight.
The dantian area is a bit more difficult to define. It is the thing that joins the hips with the shoulders and rib cage. It consists of all the stuff that sits in and on the pelvis as well as the lower back (lumber region). It is difficult to isolate because it is physically so close to the hips. It is also difficult to see how it can drive the shoulders unless you consider the rib cage as basically an extension of the shoulders. All the requirements of Tai Chi amount to two interrelated goals: to isolate the dantian and to connect the rest of the body with the dantian.
The most basic requirement for doing Tai Chi is song kua, or relax the hips, means that the muscles surrounding the hip joint, i.e. where the thigh bone meets the hip, should not be used to any great degree in supporting your structure. These hip muscles can then be used to adjust the angle of your pelvis so that your upper body can remain relaxed, or direct the jin if so required. If you haven't achieved a song kua, then everything else is academic. You will not be able meet the requirements for the rest of your body.
In Chen village the term kua is sometimes used to mean the hip joint in general. However the term song kuameans that the inguinal crease, or where the legs join the pelvis, is bent. The problem is that Tai Chi also requires you to bend your knees. This semi squatting position puts a lot of load on your quads. Your quads consist of a group of 4 muscles. The bottom of these muscles are connected to the knee cap. The top of 3 of them are connected to the thigh bone. However the top of one of them is connected to the pelvis itself and is classed as a hip flexor.
If you engage a hip flexor then you are using a hip muscle, therefore violating the requirement to relax the hips. So not only does Tai Chi want you to put a huge stress on your quads, it also only allows you to use 3/4 of the muscles available! This is why standing meditation is so painful, and why you can tell whether someone has begun to achieve some gongfu (skill) in their Tai Chi by just looking at how their leg muscles are developed.
So what is so bad about using a hip flexor? A hip flexor is a muscle that you use pull your knee towards your chest. Now if you engage a hip flexor in a Tai Chi stance, it will pull the front of your hip downwards, thus making your bum stick out and your lower back arch. To counter this, you will have to engage muscles opposing it, i.e. hip extensors such as you gluts. Now you are locking up your hip joints even more. How can you manipulate your hip if it is so locked up?
If the lower back arches then you are violating another one of Tai Chi's requirements. An adults spine looks like an S, with two arches, inwards in the lower back, and outwards in the chest area. I'll ignore the neck and coccyxs for now. "The back should be curved" means the entire spine should look like a very shallow C. This involves mainly changing the inward arch in the lower back into an outward arch. This all sounds very unnatural, but it is actually the shape the spine takes if you allow it to relax completely. If you float in a swimming pool, your spine will take this shape, and if you can sit in lotus you will achieve it too. It is also the shape of a babies spine. So the next requirement, that the upper body should be relaxed, is actually how you achieve the correct spinal shape. There is nothing worse than flattening the lower back by tensing the stomach muscles.
The shoulders too should be relaxed. But how can you lift up your hand if your shoulders have to be relaxed? Tai Chi has a saying that goes something like "you shang, you xia, you zhou, you you, you qian, you hou". This means that when there is up, there is also down, and when there is left there is also right, and when there is forwards there is also backwards. It is usually wrongly interpreted as "to go up, first go down, and to go left, first go right, etc", and this interpretation is responsible for a lot of the see-sawing movements in badly done yang style, and the excessive twirls movements in badly done Chen style.
What it really refers to is the way the body moves internally, and it should be applied when using the shoulders. To lift up your hand, you should open your body forwards and up naturally. This is called peng. However as your hand reaches about shoulder level, your peng gets used up. Lifting it up anymore using shoulder muscles will cause those shoulder muscles to become tense To continue powering your arm, you need to close the muscles in your back and side. This pulls your shoulder blades down and in, and your hand will go up and away from your body. As you can see, up and down are present at the same time, as are left and right, and the shoulders are able to stay relaxed
The requirement of holding the chest in allows the muscles in the back that are used for chest breathing to relax, allows the chest to connect with the dantian. It also allows you to isolate your diaphragm. If you engage your diaphragm your dantian will feel full but relaxed, and if you relax your diaphragm your dantian will feel soft and light, meeting another requirement. Perhaps relaxing the dantian is really another way of saying isolating the diaphragm.
In movement, it is required that the movement is generated by rolling the dantian. The dantian acts as a physical ball joint between the hips and the chest, the hips and the chest both being sockets. Most of the muscles in the upper body run diagonally across, and it is these muscles that are used to slide the body round the dantian to create the dantian roll effect.
This is my attempt to try and demystify Tai Chi. If it has intrigued you, then nowadays there is a lot more information about this all around the internet, especially sites related to Chen Tai Chi. I haven't changed this article materially since writing it in 2001.