Some people believe Ki is an actual physical force. Others see Ki as a custom that they have no great belief in, but accept in the dojo due to the weight of history behind it. Some practitioners think of Ki as a simple mental shortcut, but an extremely effective one. That is because the human brain is limited and mental constructs like Ki allow a martial artist to place their undivided attention on a single image that implements a multitude of anatomical changes.
To become a martial artist a person must completely redesign the way they interact with their body. A basic example would be the Pull Step. When a new Aikidoka is being taught the Pull Step they are often told to imagine pulling energy, or Ki, from the earth. They pull it through their feet and into their core. This image causes one to subconsciously bend their knees and tense their core and thighs. This tension pulls the pelvis forward and braces it directly under the spinal column. The imagined strain also causes a person to plant the lead foot powerfully and align the weight of their tensed leg and core above it. The back foot ends up bearing absolutely no weight and is pulled into position and planted: hence the name Pull Step. In this way one mental image, pulling Ki from the earth, implements dozens of minute adjustments without straining the mind’s ability to multitask.
That efficiency is vital. Even simple Aikido techniques require that a person maintain perfect lumbar spinal alignment, a 70-30 weight distribution on the ball and heel of the foot, a braced core, hands aligned exactly along the central axis of the body, bent knees, constant execution of Unbendable Arms, engaged hip flexors, and a slew of other adjustments. They must then add in subtle footwork patterns, precise distancing, a complete awareness of the opponents body, and, finally, the actual steps of the technique being executed.
As such it is impossible to consciously track every factor in play.
That is why dojos often use Ki to teach. Ki is an extremely versatile mental tool. Its description is vague in a way that makes it easy to shape it into an appropriate mental image for either general training or detailed corrections to a particular skill.
Some Dojo’s ascribe mystic importance to Ki, while other marital artists consider that a sign of unprofessionalism. From either perspective, Ki is a way to improve your chosen art. So why not take advantage of every possible tool? From either perspective it is useful to feel your Ki flow up your opponent’s arm so that you can lock each of their joints at their maximum range of motion and use the immobilized limb as a lever. Walk with all your weight hanging off a thread attached to your head so that your thoracic spine is neutral. Use Moe Shihan’s 60/40 principal to turn any bone into a lever. Push Ki from your palms so you don’t engage the bicep, use your index finger as a laser pointer, and try moving with all your muscles dangling off your bones like hammocks as you walk.
In the end, there is only one human body. A martial artist must learn how to have total control over both their own and their opponent’s so that they can ply the laws of physics and anatomy to their favor. Maybe you work best while imagining vector sums and centripetal force. Others might need to think about channeling the essence of water instead of being earthy and grounded. A tool is a tool. Call it Ki, Chi, Kokyu, Mudwalking, Bio-mechanics, or the ancient magic of Atlantis, just so long as it can also be called effective.