Tap into the Mind's Power
Consider the quote, "Humans use only 10% of their brain." This often leads to the question, where on earth is the other 90% and how do we use it?
I personally do not have that answer, however, I do see how some of it can be used an manipulated with regard to budo.
Catching a ball. To many of you, catching a ball now seems like child's play, however, this was not always the case. While you were first learning your brain had to first get use to objects in flight. This may not have happened the first time you saw a ball in the air, but soon, your brain got used to a object's ascent and eventual descent on the earth's surface. The next, more difficult, step is to coordinate the body and hand to be in a position to be there when the ball lands. Anyone that has ever been hit with a ball knows that this is not always an easy task.
Experienced ball catchers know that the way to catch a ball is not to stop, calculate the trajectory of the ball, decide which muscle groups to fire to activate the body, then activate those muscles to put the body in the correct postion to be there when the ball lands. This would be far too time consuming to be effective. Instead, simply look at the ball and allow the brain to do the work for you. Your brain, a supercomputer, has no difficulty doing these calculations whatsoever.
Like I said however, your brain cannot automatically do this from birth. It first has to learn the rules that an object in flight, as well as the rules that govern our body's movement.
Theoretically, if the brain had complete knowledge of the "rules" of budo (ie. principles, techniques, body mechanics), one could simply "turn off" their active mind and allow the power of the brain to do its work. That is why it is impossible for Aikidoka to replicate their moves exactly, often they don't even know what they did! Interestingly enough, Ueshiba himself said, "Learn and forget." This means to "learn" the rules, but don't try to remember them when the situation arises. You must do the prep work, but when attacked, allow your brain to do the work.
This is also no easy task, it takes mental disipline. But, one can see that some of the power of the mind can be utilized in Aikido. If you spend time training your body, why not spend at least some time training your mind?
Is this the complete answer to what ki is? No. But the mind can do such amazing things, it would seem no wonder that a word like ki might be used to explain the unexplainable.