Chris, this example is EXACTLY the opposite way of how you need to look at it, "This kind of training requires all non essential muscles to relax, and all essential muscles to fire in their most efficient order."
Flip this on it's head. Say someone is pushing you from the front. Now, in terms of efficiency, it is most efficient to completely relax the back side, and essentially use the other person as half of a supporting arch, could be a clinch or whatever, for example. If they pull away suddenly, oops, you're now hosed, you will fall forward, however slightly, in that moment. You can react quickly to pull back, but then you're reacting, and while you're reacting to their action, they've already moved on to hit you. And you can react to that hit and while you're doing that they're kicking you and on and on. You're always one step behind. You're always committed, except when you're doing nothing, and doing nothing is not an option.
You must be on everywhere beforehand, so that there's never a reaction. Reaction is too late - it means the other's initiative is dominating you. Half the game is making sure the mind never shuts off to the infinite possibilities of what can happen at any time, despite what your body may incidentally be doing at the time.
With your example, we are getting into the idea of a technique. A technique might be to relax all of your backside, and lean against a person, that might be a good or bad technique, but it's not the body use specifically.
So if you are "everywhere beforehand" so that you never have to react, how are you not using a huge amount of energy? Are you saying with being "everywhere beforehand" that your body is always working?