Good question and one that will always be asked no matter how much it is talked about because it is intriguing ki or qi. Chinese at a time had a huge supernatural superstitious look on things (in this case martial arts) as well as some practical discovers to explain what they discovered and what they didn't understand. Things we now understand through science and its language. Taoism comes to mind. Chinese medicine has a huge impact on Chinese martial arts (kungfu) development, and it too was explained and theorized through taoism. Those ancient Chinese doctors debated to be mystical or real people used a language with in the context of their time, ancient language and culture which also had taoism had an influence.
Now simply many look upon taoism as a religious belief because of the supernatural and superstitious concepts it held. Now taoism wasn't completely such, it also had practical observations like that of the famous Lao Tzu, whose compared to ancient wisdom/psychology/self-help/life coaching/interpersonal things and explanation of life, really absent of any deity and customs denoting a religion. But, taoism did develop a limb that developed a supernatural and superstition side to it. But these early Chinese doctors felt exercise and breathing where key to fending and curing off disease.
One such Chinese doctor provided the world with much of the medical knowledge we have of toxicology, fevers and other common aliments. There are ancient stretching like and warm-up type of exercises developed that some say where the basisi of Chinese martial art. Bagua and they trigram are credited by one doctor who is said to base those things on walking in the pattern of a tortoise shell. Who also felt breathing and stressed the important movement and breathing had to health. At this time the supernatural and superstitious side of taoism was not around, so medicine and health was not influenced, that came later. You have to put this into context. In ancient times, prior to these doctors in China the thought of breathing and exercise wasn't seen as an important part of keeping us healthy as it is today. They knew it was important but they don't know what we know today. And at that time the described it the best they could. One term is in the Chinese language Qi for breath that gives us what the ancient Chinese doctors said was the life force which is mobility and the effects exercise has on combating illness and promoting health. Chinese medicine was in it's infancy and prior to these doctor who where the first to really make head way into medicine. They were pretty practical and in agreement that there are benefits to the body to do exercises and breath. It is my guess it was an influence from Yoga. Hence the credit of the Bodhidharma to Kung fu. And the character in Chinese for Qi is a pot with a fire under it and the lid being lifted off. Which indicates steam/air as a result of movement.
Now enters the more colorful language and view of the supernatural taoism influencing how Chinese see everything. It works really well developing creation myths that are supernatural as well as superstitions. People are explaining things at this time what they don't understand through supernatural concepts and language and superstition. Now the practical idea of Qi or breathing becomes much more, the idea of life force/moving to stay healthy becomes something of the supernatural power as martial arts hone their skills and explain through ancient medical terms. All the while Chinese martial arts skills become spectacles of entertainment, much like a circus side show. The benefits of health and protection of kung fu is overlooked at that time. So of course you get something pushing someone off balance and they are lifted by the force of their feet, i.e. Bruce Lee demonstrating his famous 1 inch punch, or punching someone hard enough in the chest to make the heart stop on lookers amazed deem it as a mystical magical supernatural feat. Here is an opportunity for a not so ethical martial artist to run with it and claiming supernatural powers and abilities. I even see that still alive and well today.
Qi is the act of inhaling and exhaling and movement/exercise. Breathing is essential to all physical activities. More oxygen to the body the better performance. Sport medicine has told us that for decades. It is no different in martial arts, if you are winded your screwed. If the air gets knocked out of you, your screwed. Just as then as now. Qi isn't a magical Dark Side or not force that can be commanded at the will of a tried Jedi or their religion. My taiji instructor does some weird things that over the years I have scratched my head, and said, that has to be some kind of magic. But the truth is, it isn't magic, it isn't a invisible supernatural force called qi that sends me flying helplessly across the room with just a slight push on my elbow. It is my lack of understanding of the mechanics of the human body and physics. My teacher says the same thing. Qi is breath and movement that is about better health resulting in better performance and not magical powers. He should know he is a western and eastern doctor who has study both kung fu and taiji for about 45 years. If anyone says Qi/ki is supernatural and more than just explained...run.
