In fact, anything you do with the intention of benefiting your energy can be considered Chi Kung. And all Chi Kung contains common principles - mind, eyes, movement and breath. Another way to express this is - the mind is the presence of intention, the eyes are the focus of intention, the movement is the action of intention, the breath is the flow of intention. These are the "secrets" of Chi Kung - and they are often taught at the beginning of training. Of course it takes years of exploring these ideas in practice to begin to truly grasp their significance.
Kung Fu (Gung Fu) means "Progress Achieved through Discipline." Discipline means learning through action. The word discipline in English is related to the word disciple - one who learns. In the West the idea of Kung Fu is commonly thought of only in terms of martial arts. In fact the term applies to any activity in which paying attention and practicing to improve are important - in other words just about any worthwhile aspect of life.
Nui Kung (Nei Gong) means literally, "Inner Cultivation," and refers to the subcatagory of Chi Kung exercises which increase access to energies commonly outside of the human experience. Obviously, if it is possible for humans to access these energies, they cannot truly be outside of human experience, perhaps only outside of commonly perceived experience. Different sets of exercises focus on different specific aspects of experience, including altering states of consciousness, opening to universal energies, and manifesting unusual abilities and powers. Many of these exercises are considered dangerous if taught to the unprepared and all of them must be treated with an appropriate amount of respectful attention to avoid possible difficulties resulting from their practice.
If you are going to cultivate Chi and apply it in healing work with others there are important precautions to heed. The most important of these is to remain clear of personal attachment to the process. Nothing will deplete energy faster than seeking ego gratification with the "power" of your healing work. Fortunately for most, these tendencies usually get worked out early in the process, particularly if they are working with a teacher who is themself clear.
There are also numerous levels and depths of this healing ability. Certain healers using externally-directed Chi seem to have better effects with some illnesses than others. Ultimately it may be that to truly master the art of directing Chi in the healing process you must develop a wide spectrum of abilities and be able to apply them in a variety of types of imbalances in body, emotions, mind, soul and spirit.
Finally, although you may find books or teachers professing to teach you the secrets of Chi Kung and the power of it's applications, it always comes down to what you, yourself are willing to practice and experience. The layers are there for you to explore when you make yourself available for the learning. At the same time, it doesn't hurt to learn whatever you can from those who have stepped through the doors before you. In the words of the Zen poet, Basho, "Do not seek to follow in the footsteps of the men of old, seek what they sought."
(c) 1995 B. Eichelberger