Chi and the martial arts
The mind controls the chi, the chi controls the strength. So runs the traditional law governing the use of chi in the internal martial arts. The body requires a minimum amount of vital energy for our daily lives. This you could call 'core energy'. Any extra energy is stored within the body and added to your potential strength. This internal strength can be trained and brought under the control of the mind.
The relationship of the mind and the chi is somewhat akin to a rider and a horse: the mind is the rider and the chi is the horse. Once the horse is trained it will go wherever the rider commands. After consistent, correct training, the accumulated chi can be led to any part of the body in a fraction of a second, by the mind. Thus when you are performing the tai chi form, pushing hands or self-defence applications, you first form the idea of doing something a split second before you do it. Instantly your energy will lead your body to perform the task required.
If you doubt this mind-body relationship, perform the following test. Try to walk forwards while you are thinking about standing still. You will find that you cannot move; all the actions of the body are governed by the mind. The mind forms the idea of the task you wish to accomplish, and the energy sends impulses to the muscles and tendons to move in a prescribed way. Thus, when you wish to deliver a powerful strike, you command the body to do so, sending a surge of force to the hand or foot. When chi is concentrated in this explosive form it is called Jing (Ching) or 'essence'.
The inter-relationship between the mind, body and Jing might be explained best with the analogy of a car. The chi you can think of as petrol, or the prime mover of the car - without it the vehicle could not run. The body of the car is the human body, with the wheels representing your arms and legs. The driver and the steering wheel represent your mind and the centre of tan tien. Wherever they direct, the car will follow. Jing is the momentum of the car, and the potential explosive force now inherent in it. Therefore Jing is the total force of the combination of all of the factors involved. Chi is not Jing - It is the prime mover which helps to create this explosive energy.
Source: Yang Tai Chi Chuan, by John Hine.