Kime Waza.

Developing Kime, in my view, is the single most important thing one can do in the Martial Arts, regardless of which art you study.
Depending on how much time one devotes to the training, it may take years, but it may not, I don't think anyone can really place a timeframe on such things. Notions that are preconceived must be left behind in order to fully comprehend such a thing.

There was a video that Shinzen shared on Facebook, the second part of an old Shotokan training video in which, at one point, the narrator says something about Kime as the demonstrator performs stationary punches in the air.
It was a very vague statement, and quickly shifted to muscle tension in the practitioner without further explanation or ellaboration.

One way I think of it, one way I practice it, is stationary. I take a Kongo Zen approach, as you find in Shorinji Ryu Kempo that was founded as a Meditative and Spiritual Art in Japan after World War II.
It doesn't really matter the source, what matters is the method, and I find that it has helped me immensely in my training in developing the Fudo-Tai and Fudo-Shin principles, which are essential in Kime Development.

You sit in Seiza and pour every last ounce of energy into focusing the mind and body as one unit, not worrying about all the spiritual stuff that goes along with it, just focusing. You focus on holding your body as still as possible, like a stone, you focus on being conscious, but relaxed, alert.
Furthermore, you get a single image in your head and you hold it there. What I do is I imagine looking at myself from the outside, watching my body, so that the act of consciously keeping my body still and breathing are also a part of my mental focus, the two become integrated.
The mind should not wander, thoughts are definitely coordinated.

Do this for about fifteen to twenty minutes in the mornings, find a method that works for you, but remember that it is about being grounded and focused, with effort in every part of your being.
Not really a whole lot of difference between this and what most people call Zazen, they are the same thing. Not really much else I can put into words and continued practice WILL yield something profound.

From here one can more it into the practice of moving principles, applying the full mind-body unit in every single act, not just Budo.
You can tell the difference between someone with strong Kime and weak Kime by their physical movements - again, I watched a fight one night and was able to tell who would win the match before the first punch was thrown just by the way each fighter carried themselves.
This is nothing special, nothing really to brag about, just an illustration, however incomplete, of the principle at work, and something I'm sure those that have taken the time to put in the effort know all too well, even more than myself.