So we use the ki to let the mind in on the deeper secrets of the body and in that way, through serious effort, the mind "becomes one" with the body because it can enter any place in the body through the ki.
When the mind and the ki work together, the mind can penetrate the entire body and can recognize where connections are too slack.
To finish the whole thought, when the mind becomes that wise to the body and ki, it is fit to direct them and not before. We can do many things in the name of harmonizing and directing ki and the body, but most of them are fanciful because the mind really floats along in confusion about what ki even is and what its flow is like. The only way to know is to apply the mind to observation of the ki for a while. Sometimes this is stimulated by stressful exercises and the softest can be the most painful for the mind--pain being not the actual feeling but a kind of friction set up by the mind trying to escape from the serious focus.
When the mind fully penetrates the body and ki and is in constant harmony with them, we can seek to understand six harmonies and other things related to dantien, etc. But finally, we get to the old saying that the mind leads the ki and the ki leads the body.
Mike Sigman added something to that a while back that I think I understand now. Mind leads ki and ki leads body are only two harmonies. Aren't there supposed to be three? The first is a division of mind into heart (kokoro) and what we think of as mind. That second type of mind is purely calculational and intellectual. But the real essence of life and self comes from kokoro, which is the heart, the origin of all our feelings and desires and impulses. Most of us wage a lifelong war between mind and heart (heart or head). Some fall to one side and, with no restraint on their kokoro and its impulses, become horrible criminals. Some with no restraint on their kokoro become saints, geniuses...insane. So it is vital that the rational mind have final sway over the wild and powerful heart, to tame it like a lion domesticating itself. Most lions now are in protected preserves for fear of their life, anyway. The result of a kokoro tamed by the rational mind is "I" in Japanese and "Yi" in Chinese. Taming one's own heart can allow one at least to walk at one's own discretion among men and not be injured nor accused. Maybe. Deep and subtle attention to ki and its movement as the source of physical action is a deep expression of this congruency of self, the mind and emotions, the mind and the spirit, the spirit and the body. One becomes truly "one" instead of a conglomeration of mysterious and maybe dubious parts.