What is Chung Chi? It is a combination of two words, ''Chung'' and ''Chi''. Let's me explain it separately as follow:

Chung, is a big word in Chinese. If you would write China in Chinese, it is ''Chung Guo'' or The Nation of Chung. The highest Tao in Chinese Culture in Chinese call ''Chung Tao''. So, what is Chung? It means: in the middle. In Taichi this means: in the middle of the bones. It also means just right, not too much nor too less.
Chi, is a term for general energy. In Martial Art, it can be divided into two terms ''Shia Chi'' and ''Li Chi''.

''Shia Chi'', the energy from fresh. It is from one's desire, from one's will, disregard the balancece between one and his surrounding. It can be powerful and destructive. Yet, it will fail by encountering something stronger and more powerful. ''Shia Chi'' has it's physical limit.

''Li Chi'' is the energy from pure theoretical being. The energy that exclude our own desire. The energy purely from the way that nature operate. Such as:

1. ''Moving object remind move, still remind still.''
2. ''Momentum, as mass multiply by velocity.'' The difficulties of change of an object's trend.
3. ''The phenomenon of pendulum.''
4. And so much more......

These theories of physics describe why and how the nature operate. It also indicate a way on how to effectively balance one object with it's surrounding. By understand these principles and manipulate it, one can apply ''Li Chi'' (The physics principles and other sciences) instead of ''Shia Chi'' (One's Desire). The delicacy and the depth of ''Li Chi'' is unlimited.
In Practice

''Chung Chi'' is a series of position for center of rotation which travels through the axis of every bone. Mentally they start from kidney, 60% of concentration follow through the spin goes up then divide into two, 30% through right shoulder to right fingers' tip, the other 30% to left. They are 40% go downward to both toes. It seems like they go to four different routs, However, their traveling operate simultaneously.

If we observe them at any point of our arms or legs or torso, movements are circling around those axis's virtually in the middle of the bone . The bones are hold steadily and rotate without friction around joints as these axis operate by ''Chung Chi'' (You can say a manner of operating). It is how ''Chung Chi '' maintaining one's integrity under opponent's pressure.
Conclusions

''Chung Chi'' is to apply ''Li Chi'' and helped by ''Shia Chi'' in a way not more nor less or ''Just Right'' in Taichi practice. When concentrate in practicing Chung Chi in one's Taichi training, his progress is unlimited. And once he can govern ''Chung Chi'' he will feel the freedom within. What he will feel is that he need not fight against anything instead of cooperating and coordinating with them.