So what is Energy / Chi / Ki / Prana?
The word "energy" on this site is a translation of the Eastern words Chi (from Chinese), Ki (Japanese) or Prana (Indian). It is related, but not precisely the same thing as the scientific concept of energy. The word as used in this context, is something that essentially should be felt.
Whereas science emphasizes objective measurement and bypasses personal observation as much as possible, this is not my starting point. I want to feel and observe things myself. Thus the power to perceive and heal stays in my own hands. This is what the concept "energy" for me is all about. This does not imply that scientific measurement of energy isn't possible, though.
When you place your attention somewhere in your body, you can feel that it's not just your perception that is focused there. Something else is happening. Energy is displaced to that area. It is possible to measure this: the temperature of the spot you focus your attention on will rise. The Chinese say that "Chi follows Yi," where "Yi" is the mind or intention/attention.
The fact that knowledge of energy seems to have developed mostly in the distant past, is related to what people pay attention to. In the modern world, we have all kinds of things that grab our attention, like TV and other media, the thousands of objects in our houses or at work, and advertising. As people didn't have all these diversions in the ancient world, they were able to pay more attention to their bodies and feelings, so were better able to notice energy. The meditation techniques that were developed, also opened the way to a precise awareness of energies.
Based on their knowledge of energy, the Chinese developed techniques to deal with it. To enhance health, Qigong and acupuncture were developed, in turn enhancing their understanding of energy. Based on that understanding, martial arts were developed, like Tai Chi and many others.
In India the chakras were discovered, and the intricate energy system supporting and interconnecting them. Here also techniques were developed to use that knowledge, like yoga and mudras. Many of these techniques entered into use in other countries as well.
Energy seems to be all kinds of things. Although it seems to be something ethereal, it may be more practical to look at it as some sort of functional relationship. A mudra is performed, an acupressure point pressed, a sound made, and one feels a different energy. One increases some energy; one feels some energy; energy becomes unblocked. The energy of the moment is allowed to affect the outcome of an I Ching oracle consultation. It's all very much a functional relationship.
One doesn't know the mechanics, but one can handle energy by feeling it.