Re: Mechanics of mushin
I recall an interview with the late Tohei Akira Shihan where he compared mushin to driving a car. When you drive a car, you don't think "I'll push the accelerator this much," or "I'll turn the steering wheel 240 degrees." In fact, you think about where you're going, and what other cars are doing, but you operate the car itself without conscious thought. So that's basically the goal - to be able to correctly express yourself physically in a conflict without conscious thought.
So let me run with that. Now imagine that you're driving a car, but you're in a hurry. Now you're still driving without conscious thought, but you're distracted by everything else. You're frazzled. Maybe you miss your exit, or you almost hit somebody. That state of mind is no good, so you need to develop "heijoushin" 平常心 - the everyday, calm state of mind.
Okay, now imagine you're driving a race car. You're going at extremely high speeds. One wrong move and your car will go flying, possibly killing you. On top of that, the other cars may be bumping you, cutting you off, and basically making it difficult for you to complete the race. What you develop here is fudoushin 不動心 - imperturable mind. You operate the car automatically - you're flowingly aware of hundreds of things going on around you, and despite the danger you are not afraid. Mushin, heijoushin, and fudoushin.
At least that's one way of looking at it.