"Keep one point" is a phrase I've heard maybe a couple of million times since I began studying Aikido. It's a simple three word declarative sentence in the form of an instruction that begs the answers to a number of questions:
What is one point?
Where is one point?
What does it mean to keep one point?
Why do I want to keep one point?
How do I train to learn to keep one point?
How can I tell if I'm keeping one point?
What is one point? - One point is mind and body that are temporally coincident (coordinated); to the point that the temporal separation of the behavior of both is too small to be consciously detected, that is, thought and deed become one. "At" one point the workings of mind and body become so indistinguishable that mind and body become mind/body. One point is synonymous with correct feeling.
Where is one point? - I was originally taught that one point was an infinitely small point about 2 inches below my navel where Ki cycled in and out of me. I could take Ki in, store it at one point and then "extend Ki" out when needed. As a starting point of my training this was an easy concept to grasp and it served me well for a long period of time. But after considering the first question above, I deduced that the "where" of one point is largely irrelevant. The explanation of "what is one point?" lead me to the conclusion that one point is a state of being and, as such, isn't located in any specific place at any given moment.
What does it mean to keep one point? - Keeping one point is my ability to maintain a very tiny temporal separation of mind and body behavior as explained in "What is one point?" above. "One point" doesn't do a particularly good job of describing the state of a unified mind and body. Mind and body are always unified. What can vary is the degree of unification that I exhibit at any given moment. I see myself depicted as "one point" as the temporal difference separating my mind and body approaches zero. Keeping one point is maintaining that degree of integration over a period of time.
Why do I want to keep one point? - Keeping one point enables me to operate at the peak of my abilities. When keeping one point I am in my most dependable and powerful state where thought and deed converge to a single point of unconscious action.
How do I train to learn to keep one point? - My vehicle for training to keep one point is Aikido. Other people follow other paths; as O Sensei said, "There are many paths to the top of Mt. Fuji". My path involves the practice of a combination of Ki exercises, technique, weapons work, resistance training etc. Specifically, my Aikido training turns my gaze inward so that I may realize my full potential. Only then will I be able to truly express the outward form of my Aikido in an effective manner. Ki exercises are performed either standing or in motion, with or without a partner and sometimes require that I deliberately lose one point in order to give me practice in regaining it while I am being stressed. Technique practice requires that uke provide me energy and resistance in correct proportions to insure I am provided with proper feedback as I execute a throw or immobilization.
How can I tell if I'm keeping one point? - We have may exercises (Ki tests) that are designed to test my level of mind/body coordination. The exercises variously challenge me physically, mentally and combinations of both in order to give me an indication as to how correct feeling is being manifest, or, as is sometimes the case, not. I can gauge my level of mind/body coordination by the amount of force I am able to deal with before my structure and stability (both mental and/or physical) become compromised.
"Keep one point" then is all of the above, and more, condensed into a simple easily remembered instructional metaphor. Only when I was many years into my study of Aikido did I begin to see the deeper layers contained within that simple sentence.