I think this article by Zhang Yun is much better than that of Dr. Yang. "Jin in Taijiquan":
http://www.ycgf.org/Articles/TJ_Jin/TJ_Jin1.html1. Li and jin
Martial art is like any other types of knowledge, it took a very long time to develop. At first,
people had no principles or concepts, only hands-on experiences. At some point those
experiences accumulated to a point where it was possible to summarize them as general
theories or principles. People can then test out those ideas. And if they turn out to be
correct, they could be used to improve the practice. Better practice in turn leads to further
understanding of underlying principles, thus the cycle continues.
It is only natural that in the beginning, these concepts are undifferentiated, simple, and
coarse, and only became clearer and more refined gradually over time. In terms of
understanding force, the first major distinction made in Chinese martial art is between that of
li and jin.
In everyday usage, both of these words mean physical force, and can be used
interchangeably. Very often, people use jin to denote a very large force. In martial art,
these are technical terms with more precise definitions. Li is simple muscular force, what we
call “untrained force”, “natural force”, or “instinct force”, because no prior training is
necessary before using it. When we contract our muscles in a simple, natural manner,
doing things like bending the arm to bring a fork to the mouth, or extending the arm to push
a drawer shut, we are using li.
Li is simple, its major attributes are quantitative: how big, and how fast.
If li is the raw, basic material, then jin is the sophisticated, finished product. It is li transformed by practice,
something that produces the desired results with higher effectiveness and efficiency.
Example: two identical twins, same size, same muscle composition – same level of li, one is a professional golfer,
the other never golfs. Stepping onto the course for the first time, the untrained twin, lacking any knowledge of the
sport, can only use what he was born with - li. If his first drive does not go as far as he hoped, his natural instant
will lead him to use more li – swing the club harder, with more speed. His trained sibling uses something far more
powerful and sophisticated. He has jin, and use it to great effect: he is able to drive the ball much further, place it
on the course with much greater accuracy, and often with much less effort.
Jin is complex, its qualities are not just quantitative but qualitative.
Li is a product of nature, jin a product of nurture. Everyone is born with the ability to generate and use li, no one
has jin until they have gone through the necessary training. As we have seen in the previous example, when we
start on an unfamiliar, new activity, simple muscular force is all we can use because we do not know anything
else. This force tends to be highly clumsy, awkward, and inefficient. For this reason in martial art we also call li
zhuo li – awkward, untrained force. Zhuo also means muddy, the opposite of clearly distinguished, clean,
polished, and refined, characteristics we commonly associate with jin. As we become trained, that li is gradually
transformed into jin, something much more effective and efficient. For this reason jin is called “trained force”.
Transforming the primitive li into the sophisticated, refined jin is the one of the main objectives of any
sophisticated martial art practice.
2. Wai Jin and Nei Jin
In the past 400 years we saw the rise of a new class of martial art. More effective than ever, their real
significance is that they represent a giant step forward in efficiency. This was made possible by the second major
development in the theory of force in martial art. By this point people had enough knowledge about jin to make
further distinctions between two main types of jin: wai jin - external jin, and nei jin – internal jin. New knowledge
gained from the research into internal jin gave birth to internal martial art. Its members are Xingyi Quan, Taiji
Quan, and Bagua Zhang.
To be called an internal martial art does not mean it uses only nei jin. No martial art can be complete without the
use of wai jin. So internal martial art just means the emphasis during training and fighting is on nei jin, with wai jin
playing the supporting role. Conversely, to be called external martial art means it uses mostly wai jin and very little
nei jin.
So what exactly are wai jin and nei jin, and why does internal martial art favors the use of nei jin? To answer that,
we will first take a look at what they are, and then how they are tied to the central philosophy of internal martial
art. As Taiji Quan is the most systematic and detailed in making these distinctions, we will use it to illustrate these
ideas.
2.1 Definition
Wai means outside or external. To be outside is to be visible. So wai jin means the type of jin where, when used,
an observer can, just by seeing it, understand every aspect of that force: how that force is stored and released,
its speed, angle, direction, etc. We are using wai jin for example when we do a quick and hard punch.
By definition wai jin is yang. Yang in Taiji is not the pure yang, it has a little bit of yin in it. This is called gang
zhong rou – hard containing the soft. The advantages of wai jin are that it is quick, hard, sudden, and powerful.
When released, the feeling is like that of an explosion or eruption, unstoppable, capable of destroying everything
in its path. It can be used to seriously injure or kill the opponent.
The disadvantages of wai jin are: generally the movements are large, and given its external nature, making it
relatively easy to detect and defend against. The duration of its power tends to be short. And once released, you
cannot easily change its trajectory or any other attributes of that force easily. Finally, the storage and release of
power here are completely separate processes.
The key points of using wai jin are speed, power, and accuracy. Practicing wai jin is about fulfilling our potential.
Some internal martial arts masters can release incredible power. It is about absolute quantity; we want to build up
as much of it as possible during practice.
Common types of wai jin used in Taiji Quan are: duan jin – breaking force, chuang jin –ramming force, cun jin –
one inch force, leng jin – cold force, dou jin – shaking force, and chong jin – charging force, etc.
Nei means inside or internal. To be inside means it is something that cannot be seen, only felt. An example of nei
jin is nian jin in Taiji Quan. When we can stick to our opponent perfectly without either struggling against or lose
connection with him, there is almost no physical movement at the contact point between two of us, yet inside our
force is having its effect. The external movement is so minimal and subtle that it escapes the eye.
By definition nei jin is yin. Yin in Taiji is not the pure yin, it has a little bit of yang in it. This is called rou zhong
gang – soft containing hard. The advantages of nei jin are that it is small, long, changeable, and combines the
storage and release process. Small means the movements required are in general very small, sometimes not
even visible externally. Long means when the force is released, the effect can continue on for a relatively long
time. Changeable means its quantity and direction can be altered during its release. The processes for storing
and releasing are one, with no clear separations and breaking points in between. The combination of all these
attributes makes it easy for us to conceal our true intentions. With nei jin, making accurate determinations about
its amount, direction, angle, direction, etc, are very difficult, making it much harder to defend against.
The main disadvantage of nei jin is that, although you can use it to move people easily, it is not powerful enough
in itself to be truly destructive.
The key points of using nei jin use are changeability, subtlety, and concealment. Practicing nei jin is about
developing more subtle and refined skills. Nei jin is more about the relative than the absolute. It is qualitative in
that it is more about how to control and use our force. Most high-level skills in Taiji Quan come from this type of
training.
Common types of nei jin used in Taiji Quan are: chang jin – long force, chen jin – sinking force, zhan jin –
adhering up force, nian jin – sticking to force, lian jin – linking force, and sui jin – following force, etc.