Qi is synergistic function as it is perceived by the aspect of our consciousness that is responsible for intuition. It is often spoken of as energy, and elaborated upon with metaphors of water, but this is merely to facilitate explanation. Energy is capable of transforming into matter and back again, but qi does not do this. It also does not register on the electromagnetic spectrum. Indeed, yin and yang qi is understandable as the positive and negative aspect of magnetism.
The very first line of the section of the Dao De Jing that talks about the Dao (“The Way”) says that “the way that can be spoken of is not the true Way.” Or as Alan Watts puts it, “The course that can be discoursed is not the true Course.” The same holds true of qi. If we think qi is anything like the metaphors we use to discuss qi, we miss the point entirely and go clamoring after illusions and fancies. The best way, in my opinion, to understand qi is experientially. This is not satisfactory for a discerning mind of science, I know, and to this objection there are two things I can say:
1) There are phenomena which are capable of being experienced and understood in a purely rational manner, and then there are the phenomena which arrive only through intuition. Barring the notion that intuition is the result of complex lateral calculations and processing we cannot yet understand, which is still speculation despite its deference to reason, intuitive phenomena deserve to be treated as possessing categorical differences from rational ones. In this approach, qi is a quality of intuition that communicates with rational phenomena, but is not itself of reason. Our senses and mental processes grounded in reason cannot perceive it, but this does not mean it is not there. There is a rational system for qi to follow, yin-yang theory, which it always follows. Thus, qi itself is perfectly rational in functioning, though it is something outside of reason. A reasonably unreasonable phenomenon.
2) Assuming intuition is a category of rational thought that we are yet unable to trace and understand, qi would then be an imagined variable that allows a system to work. Eastern medicine relies on qi, which may be how the ancients explained nerves and hormones, which for some reason are capable of being influenced by putting needles in the body at certain places, which do not have to be the same two points for the same results.
My study of this topic, both written and experiential, has led me to a conclusion with much more in common with the first point. I invite skeptics to give these healing arts a fair shot by placating their inner critic with one or both of these points. Belief or acceptance of qi has nothing to do with their efficacy, but it is a fascinating area of study.
That said, acupuncture, Eastern herbalism, Tai chi & qi gong all operate on the level of qi. Yoga describes this as prana, which is translated as breath energy. Try some daoist breathing and see where it takes you!
Kelsey Healing Arts 
“Assuming intuition is a category of rational thought that we are yet unable to trace and understand, qi would then be an imagined variable that allows a system to work. Eastern medicine relies on qi, which may be how the ancients explained nerves and hormones, which for some reason are capable of being influenced by putting needles in the body at certain places, which do not have to be the same two points for the same results.”
When I think about this, I’m drawn to the two concepts of dynamic equilibrium and control. There’s also the concept of flux there, because there’s something flowing and something oscillating, like a wave.
To articulate the relationship, dynamic equilibrium is a condition where two variables interact in such a way that if you perturb one of them (say you increase one of the variables somehow) then the system responds so that it opposes and undoes your perturbation. It’s an active equilibrium – if you try to move away from it, something pushes you back.
For example, prices are supposed to be in stable equilibrium (in free markets), so if demand increases then prices increase and that pushes demand back down. If prices go down then more people think it’s a good deal and they buy.
The way that I understand it is that the yin/yang symbol, with the white dot in the black part and the black dot in the white part, symbolically represents the arising of those forces which keep a relationship in dynamic equilibrium, so that when there’s extreme darkness or extreme light (price too high or price too low, for example), then a source of light or darkness respectively (downward or upward pressure on the price), will come into effect, to restore the balance.
There’s a rhythm which appears out of the dynamic equilibrium. Prices go up; demand goes down; prices go down; demand goes up; prices go up, and it repeats. I think that this abstract rhythm is the “flux” of the interacting relationship which people think of when they think of qi. It’s a real oscillation that happens with any interaction which spends some time in dynamic equilibrium.
Interacting with it properly is also what gives you control. The classic case of control is somebody in the shower who thinks it’s too hot, so they turn the knob towards cold, and it doesn’t have any immediate effect so they turn it further. Then it’s too cold, so they turn it up. If they haven’t been in this situation before, they might turn it up too high, and then get too hot, and then turn it down too low, and then get too cold, and so on, until their skin gets all wrinkled like a prune.
A smarter interaction with the rhythm – namely, trying to dampen the oscillation by turning it a smaller amount each time the direction changed – would have made the showering experience much more pleasant, and more importantly it would have established control over the environment by the showering fellow.
So the other aspect that I think is there is that mastery of the rhythms, or the qi, gives the acting individual control, and even though the two variables stay in dynamic equilibrium, and both variables keep oscillating, control can shift from one variable to the other in accordance with which one has mastered the rhythms, or the qi.
Maybe showers and prices aren’t the best examples to illustrate it, but at least they’re sufficiently different from the usual examples that if you think they’re real examples then it shows a greater range of applicability of qi than you usually get.
Brilliantly put, Rune!