Timothy WK

Well... I'll jump in and try to explain how I understand "ki". I believe all of this has been said elsewhere, though I'm not sure if it's all been put together like this. Be warned that I'm still a beginner, and that most of my knowledge is still academic. I've experienced some stuff, though my ability to demonstrate it practically is still pretty limited.

As I said, I use the word "ki" to discuss and describe various biomechanical processes. But I think to really explain what ki "is", I need to step back a touch and discuss the bigger picture of what's going on. And to that end, I think there are 5 contributors to the "ki phenomenon":

Posture and Body Alignment
So the easiest thing to discuss is posture and body alignment. We all know that our bodies depend on our skeletal system for support, and that proper posture and body alignment is necessary for the efficient transfer of energy (in the physics sense). In and of itself, not a big deal, though I think many martial artists don't spend enough time thinking about the subtleties of their "structure".

It should also be noted that proper posture and body alignment facilitates a number of other "ki"-related biomechanical processes, by allowing the body to work the way it was designed to.

Relaxation
We also all know that relaxing the body improves efficiency. But again, even though most will pay lip service to relaxation, few martial artists seem to actually practice relaxation.

When it comes to "ki", it should be noted that relaxation facilitates a couple of other relevant biomechanical processes, also by allowing the body to operate the way it should.

Blood Flow
Now we're getting into the real phenomenon of "ki". One of the things that relaxation and proper body alignment does is increase blood flow by opening up pinch points in the circulation system.

The big thing increased blood flow explains is the increased body heat one experiences from "ki". This is how monks can go meditate in the winter and melt all the snow on the ground, though they're not moving. (It's quite amazing how warm one can get from simply opening up the circulation system.) Some practitioners claim to be able to generate extreme levels of heat, particularly in the hands. I haven't experienced that, so I can't say if blood flow alone explains such abnormal levels of heat.

I think this increased circulation also helps explain some of the reported health benefits from practicing qigongs and such.

Mental Relaxation & Meditation
Many ki exercises have a meditative quality about them, and meditation has been shown to have all sorts of psychological benefits. Increased focus, awareness, peace of mind, etc. I will add, though, that being mentally relaxed allows one to notice subtle sensations in one's body, which facilitates the development of some of the other biomechanical processes I'm discussing.

And the big one, the Fascia
The above stuff I've mentioned is all pretty "normal". They have benefits, but don't seem to give people those extraordinary "ki abilities" like Ueshiba or Takeda. As I said earlier, I subscribe to the theory that "ki" skills involve the utilization of the fascia.

"Fascia" is a somewhat generic term for various types of connective tissue in the body. Tendons and ligaments are considered fascia. There's a layer of fascia under the skin that holds the muscles and organs and other internal structures in. And there's fascial tissue interwoven with muscle fibers inside the muscles.

Western medicine has found that there's various channels of muscle/ fascia/ tendons that run throughout the body. It is my belief that "body connection" & ki-based movements involve exploiting these muscle/ fascia/ tendon channels.

How exactly isn't known, though there are a couple possibilities. One is that practicioners may learn how to contract the fascia itself. Western medicine has evidence that the fascia can contract, though it doesn't normally. Another possibility is that these channels are used as "strings", such that practitioner can use one part of the body to literally move another.

(If I may make a quick aside, something that's interesting about "using ki" to power movement is that there's a distinct feeling of "expanding" or "pushing", compared with the feeling of "contracting" or "pulling" with muscle. It's like... um, using your chest to push your shoulders back, compared with pulling them with your back. I'm not sure if this is just a mind trick, or if it says something about how fascial movement works.)

Regardless of how exactly it's done, learning to "connect" the body via these channels seems to dramatically increase one's power, while simultaneously decreasing the perceived effort. My understanding is this:

When using normal muscular movement, each joint acts independent of one another. Thus you use a coordinated chain of muscles. And as we know, a chain is only as strong as it's weakest link. Or in this case, one's movement is only as strong as one's weakest muscle. Technique compensates for this with proper body alignment, but the point still stands.

Now though, if one can utilize these muscle/ tendon/ fascia channels to unilaterally "connect" each joint along the movement path, you can eliminate any weak spots. Each joint will be just as strong as the next. Further more, if the "connection" is a uniform "string" rather than a "chain", it's possible to transfer the strain from one joint/muscle down the line to a stronger joint/muscle. This means that practitioners can transfer force not just through a straight line, but also through seemingly weak or awkward lines or angles.

So the reason advanced practicioners seem to have such "effortless" and "superhuman" strength is "simply" because utilizing these channels provides more efficient support for transferring force through the skeletal system. Practitioners can more effectively put their weight into strikes or throws. Also, as Mike said, utilizing these channels allows them to re-direct force, via their skeletal system, to/from the ground. This is what makes certain individuals "unmovable."

At this point, one can begin to understand the idea of "moving with one's center". The abdomen acts as a nexus of sorts for these fascial channels, connecting the upper and lower portions of the body. This means that it's literally possible to use the muscles and other internal structures within the abdomen to power and control the movement of the limbs. Additionally, the diaphragm (aka one's "breath") can be used to "pressurize" the abdominal area and add strength to these "center"-based movements.

This helps explain the highly coordinated movement of people like Kuroda or Ushiro. Instead of simply coordinating several individual movements, they are quite literally making a single movement within their centers that's "pulling along" the limbs.

Some of more woo-woo ki stuff might also be explained with the fascia argument, I believe. As I tried saying earlier in this thread, many of the seemingly mystical stuff attributed to ki---floating, inflating, invisible ki "balls", wind/energy passing by the skin or projected from the hands, the meridian lines, etc---accurately describes the feeling one gets during ki exercises or meditation (and I've personally experienced the sensations I listed). And it should be noted that many of these sensations seem to manifest "just under the skin".

I think this potentially explains why certain individuals can "harden their skin", such as when the shaolin monks bend metal bars with their necks or bend swords pushed into their stomachs.

The fascia theory might explain the increased sensitivity of high-level martial artists. Certain Taiji teachers, for example, claim to be able to feel every bone in their bodies. One of the layers of fascia (I forget which ones) contain certain nerves or sensors that report the position of the bones. If the fascia is developed/strengthened, would these nerves/sensors also be effected? (If that's true, I wonder if there's a connection with various masters ability to make "spontaneous techniques"?)

Western science has also shown that the fascia can generate a magnetic field. Is it possible to harness and direct this ability? There are stories of ki masters lifting knives or other metal objects with their palms. Is it possible to project this force, such that others can feel it? Is it possible to develop this ability and generate electricity (like John Cheng, maybe)? Who knows what's possible, though such abilities would surely be out of reach for all but a handful of elite individuals.