Most people know this, and you can see it demonstrated any place you care to look. If the guy on the foul line has his shoulders up and tight, you know he is going to toss a brick before it leaves his hand.
Much of training for physical things -- and I'm including martial arts here -- involves trying to achieve this state of just-enough between too-little or too-much.
There are a number of ways folks go about this, but I'm going to speak to two: Repetition and visualization.
You can only walk or ride a bike past the stumbling or wobbling stages once you have done these enough so that you don't have to consciously think about what you are doing. Yes, you can put it on manual and take over, but most of the time, autopilot is better.
Do it enough, you figure out how it feels when you do it right, and you don't need to worry about the kinetics or biomechanics or how gravity works, you just know.
Do the moves often enough and when the punch comes, something will be there.
There are guys who are much into the OODA Loop (Observe, Orient, Decide, Act), but that's only how they explain it, not a conscious and deliberate pattern they go through by the numbers when somebody jumps out of the alley and goes booga-booga! in their faces. That might be what is happening -- and that could just as easily be SSCG -- See, Set, Choose, Go -- or SOGB -- Shit Or Go Blind -- any other combination of words that tell you what is unconsciously going on. The words are the map, not the territory.
When push comes unexpectedly to shove, cognition is stagnation.
If you think, you stink.
Yeah, yeah, we can wrangle about that, but this is what I believe, and I'm on the podium, so I get to finish.
So, you go through moves over and over until you can react to a stimulus without having to know about how the universe was originally formed and then zip from the Big Bang to the task at hand.
Another way to get relaxed, coupled with the practice of waving and stepping, is visualization.
Back when I was doing a brief stint in aikido, thirty years or so ago, there were a series of exercises designed to demonstrate this, such as the unbendable-arm, the unbreakable circle, or the too-heavy-to-lift tricks.
Aikido didn't invent these. There were people doing vaudeville routines a hundred years ago who showed the dead-weight versus live weight stuff quite well. One smallish woman who would stand there and have two large men from the audience try to pick her up and grin while they failed.
As I recall from my aikido days, there were four ways to achieve "dynamic relaxation." 1) Keep one point. 2) Relax completely. 3) Keep weight underside. 4) Extend ki. These were different ways of looking at the same thing, actually, and when you meditated upon them and were able to keep your focus, you could do some pretty impressive stuff with your strength and balance.
Somewhere in the distant past, I came across an article -- a master's thesis, if I recall correctly, from UCLA, that spoke to why such things as the unbendable arm work, from a biomechanical sensibility, no ki involved. There's a tai chi adept I used to see posting online who did some videos showing how rooting worked from a mechanical engineering viewpoint. He also offered a teacher's test, which, if applied to a potential instructor, would tell you if the guy had any real chops. Fascinating stuff.
The aikido visualizations were great as focus tools. "Imagine there is a steel rod as big around as your arm coming from your elbow and buried deeply in the ground. It runs through your arm to your hand, where it branches, so that each of your fingers is a steel rod extending into the ceiling and through the roof. Your arm is held in place by these bars, which are far too strong for any man to bend . Keep this in your mind while I try to bend your arm ..."
The thesis said, "Relax your arm and channel all your focus into your triceps; allow no tension in the antagonist muscles of the biceps ..."
Same effect, if harder to visualize initially for most folks.
Once you knew what it felt like, you could do it without either set of props.
The problem I had was in keeping the focus when somebody was boxing my ears, even in a controlled environment such as a sparring match. Like that Mike Tyson quote -- Everybody has a plan -- until I hit them -- the ability to hold onto that thought was iffy as soon as the dance got active. To get to the level where you could maintain that focus would require a great deal of comfort in one's skills. I think you could do this, but it would be no small task. (And we aren't going into self-hypnosis and the like, which can be helpful.)
I have come to believe that visualization, like djurus or kata, is mostly a training tool. That if you can use it and the repetitions to achieve a relaxed pattern of movement, then that is where it serves best. Just as you won't do a djuru or kata in a real fight, neither will you have the time and wherewithal to do an imaginative visualization wherein your opponent goes flying when you tag him. Unless you have done it so many times you don't need to think about it, in which case it's not a visualization anymore anyhow.
Which brings up the old reliable standby, the Multiple P-Principle: Proper preparation prevents piss-poor performance ...
Now, what that preparation is is a horse of a different color, and all of our mileages are going to vary on that ...