The answers to the question What is Qi power and has it been proven to exist scientifically? mostly address the superstitious, popular notion of qi you might find in a Tarantino movie. But the word qi (or, in Wade-Giles, chi) has been used by application-oriented martial arts classics, and in particular is used in two of the criteria of Dai Longbang's defintion of the internal harmonies (Longbang 1750):
- The intention harmonizes with the chi.
- The chi harmonizes with the movement.
People who agree with Mike Sigman that this characterises the relationship between the intent and movement that is the hallmark of internal martial arts understand these verses as asserting a concrete, operational criteria that a martial art can satisfy or fail to satisfy.
Sigman has tried to give biomechanical explanations, e.g., he tries to explain qi (Sigman 2013) as our sense of something a bit akin to the potential energy you might find in a spring that can be used to draw in and push out from the ground.
But what reason do we have for supposing that this is (exactly) what Longbang meant? An oral/training tradition, or are there more explicit sources?
(Longbang 1750) - Source with citation in my answer at What's the difference between Internal and External martial arts?
To try to get a sense of what I mean by operational meaning of qi, and pushing my grasp of the Chinese terminology to the limit: in taijiquan, we talk of applications are driven by jin skills, which use body strength and create qi. In Yang-style taiji, push hands train the 4 basic jins (peng/lu/ji/an) and the forms train 4 further jins. All applications use these jin skills in application and Yang-style taiji is internal because all of its techniques use these jins which are all internal jins. Chen-style taiji is close to Yang style, but Chen people tend to talk about training jin in relation to silk reeling, which is about training peng jin: Sigman talks here of peng jin being the core jin: when you've trained this jin, the other jins are much easier to train. And part of training these jins, involves 'listening' to the chi we get from using this jin strength.
To see in one dialog both this applied use of qi together with the more metaphysical use and traditional medical uses of the term (TCM), look at this interview with Feng Zhiquang from 2000, who is a high-standing Chen-style practitioner: when he says "So practice qi instead of li [muscular force], and practice yi instead of qi. If you practice li, it will break. If you practice only qi, you will be stiff. It will flow if you practice intention [yi]" he is talking taiji training in the terms from Longbang's verses. When he says "Human beings need the balance of yin and yang to keep healthy. If yin and yang are not balanced in the human body, there will be a disorder of qi and xue(3) [blood], manifested by the disease of limbs and organs" he's not talking about six harmonies, he's talking TCM.