This process can be undertaken entirely by "the mind" but its nature does not allow it to reach directly into the body. If the mind attempts to understand the body, it has to work within its own rules. It does some things and doesn't do others. For one thing, the mind does not "feel." This is an important place where we can clearly distinguish mind and ki. The ki, in IP/IS, is used to "feel" within the body without having to have some nerve impulse. The mind can detect both nerve impulse and the "feelings" of ki. Usually, people believe that it is the mind that is feeling, but it is not. The nerves feel and the ki feels, but the mind can only receive the impressions from those feelings and it organizes that information and takes action on it.
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.
Going beyond just this basic outline sketch, you need a real teacher to show you directly what's what but I think this is a good entre for putting these concepts into perspective and place.
That's the clearest, most succint description I've read. I've mentioned before, but you should really read Tatsuo Kimura's book on aiki. What he writes is uncannily similar (albeit much more vague).