Jing, in the sense of the San Bao, is the most refined form of Qi; it is life essence, sexual essence. Even though Jing is recognised as being Yin in relation to the other forms of Qi, like all else it has both Yin and Yang aspects - the Yin being the reproductive fluids and the Yang being the saliva. Jing has the qualities of growth and development that gradually increase during childhood, reaching its height between the ages of 20 and 25. The gradual loss of Jing is associated with the ageing process, particularly with osteoporosis, reduced immune system, grey hair, loss of libido, and memory loss.
Although the level of Jing cannot be raised above its original level, practising Qigong and living a balanced and healthy lifestyle can augment it, slowing down its decrease.
In Chinese Medicine it is said that the Jing creates the marrow, this includes the grey matter of the brain "the sea of marrow"
There are three external sources of Jing: It is inherited from our parents (genetic strength or weakness); it is extracted from the refined qualities of food (Grain Qi); and it can be absorbed from one's sexual partner when practising Taoist sexual yoga.
The Jing is stored in the kidneys, the Lower Dan Tien, and the Exceptional Vessels.
N.B. Jing is also used to described "directed Qi" - this is when the Yi (Cognitive Mind) is used to focus or project the Qi as in an Internal Martial Arts strike.