This is the main principle. Mind moves first and body naturally follows. To understand the relationship of mental and physical movements you practice to imagine the future one second ahead.
E.g. you decide to lift a coffee cup from the table and your body acts acording to this decision without an effort. You don't have to think actively what movements your body should do - you can just perceive your body performing this action. This is the natural relationship of mind and body.
Ki is an everyday concept, not mystical or religious. Ki is simply understood here as coordination of mind and body. In simple terms it means that your mind and body are occupied with the same thing.
Ki is personal, not social. If your ranks, job, family and posessions are taken away from you, the only thing you are left with is your mind and body, i.e. your Ki.
Ki comes from the inside, not from the outside. In other words, nobody can give or sell you more Ki. You are solely resposnible of developing it yourself. On the other hand, nobody can take your Ki away from you.
Koichi Tohei enlisted four principles of ki development. In fact, these are just four perspectives to one principle. If you get one principle, you automatically get them all - if you loose one, you loose them all.
1) Keep One Point
2) Relax Completely
3) Keep Weight Underside
4) Extend Ki
These can also be summarised in two: relax (body rule) and focus (mind rule).
We can only see other people's bodily movements, but we cannot see their minds. That is why we cannot see Ki. There is a way, however, to perceive other person's Ki. This is called "ki-testing".
In a ki-test below the tester pushes/pulls the testee's hand to feel if the testee's mind and body are coordinated.
Ki-test 1 "pushing/pulling hand"
(testee Esa Lilja, tester Rainer Varis)
(photo by Kimmo Räisänen 2012)
The testee should keep their body relaxed and focus on their centre of gravity ("one point"), an imaginary line on the foreside of the body, or virtually anything else than on the test or the tester, who should be ignored completely. In this way the tester should not be able to push/pull the testee out of balance very easily. If the testee's body is stiff, or if their mind is wondering around, they would loose balance right away.
Needless to say, added enough force, everybody looses their balance, but this is not the point of the excercise. When a train is coming, anybody with any common sense steps aside from tracks. The point is in which way you do it - maintaining your mind and body coordination or loosing it in panic.
The role of the tester is to help the testee to find one's limits. That is why it is important to start testing gently and gradually adding force, rather than using full force right away. Sometimes it is also good to make the testee fail on purpose. This can be done by e.g. pushing very strongly, or disturbing one's mind with words or gestures. Then the testee should perceive movements of one's mind and body, especially in the moment of loosing their unification.
With some practice, anybody can keep their mind and body unified for longer and easier and in different bodily forms.
It is easy to perceive and copy bodily forms from other people, because the body is concrete and visible. However, the mind is abstract and invisible. It is impossible to get inside the minds of others. This is why we have to use mental images and metaphors from daily life and concrete world to try and understand how the mind works.
For example, in test 1 above one could imagine that one's hand is like a water hose with water running through with enormous pressure from the "one point". Or, in test 2 one could imagine that one is lifting the ukes and not the other way around. All this is of course purely imaginative, but may help one to find the correct state of mind, and bodily relaxation.
Ki-test 2 "lifting the body"
(photo by Kimmo Räisänen 2012)