Mushin is a term in Japanese martial arts whereby the practitioner has reached a state of "no mind". "Mu" means negation, or "no". And "shin" means "mind". It is a state of mind whereby the practitioner is able to instantly react to an opponent without the conscious mind getting in the way.
Most of the time, the conscious mind is busy thinking about what to do next. We like to strategize and plan. "If he does this, then I will do this thing in response. But if he does that, then I will do this other thing." Etc.
While we're thinking and planning what we're going to do next, our attention is taken away from that part of our mind we call the subconscious. The subconscious part of our minds is always listening. All of our senses go through the subconscious mind first: hearing, sight, touch, taste, and smell. And it operates independently of the conscious mind. It takes in all of these senses and figures out if there's a reason to alert the conscious mind of danger. And so, we martial artists need to allow ourselves to consciously pick up on the signals that our subconscious is sending. We can't do that if we're too focused on whatever we're thinking at the time.
There's a delicate dance between the conscious and the subconscious. Where one starts and the other begins is sometimes blurred. It becomes more clear in certain situations. When you need to pee, for example, the conscious mind tells the subconscious mind to release the bladder, but it's the subconscious mind that actually relaxes and opens up one of the sphincter muscles stopping the flow of urine in the urethra. You can't just "will" it to happen. Which is why you might have had trouble peeing when others are in the bathroom looking at you, or there are loud noises and lots of movement nearby. Your subconscious mind is watching everything and has determined that it's too dangerous right now to allow you to start peeing. You're very vulnerable when you're peeing, and the subconscious mind knows it. But your conscious mind can't just tell it that everything is okay. That's because your subconscious mind operates independently and is free to reject whatever your conscious mind tells it! You think "you" are in control, but it's really your subconscious that's the one that's mostly in control.
What does this have to do with martial arts?
Imagine you're grappling with someone in a BJJ rolling session. This guy is always able to beat you. He's bigger and stronger. He's a purple belt, and you're a white belt. When you go up against him, your mind is racing. You are trying to think 3 steps ahead. You're strategizing. You're planning. Yet, none of that ever seems to work.
I actually go over this exact scenario here:
How to beat a vastly superior opponent?
In short, don't think. Don't plan. Just relax. Listen to your subconscious. Suddenly, you find yourself moving almost as if without any thought whatsoever. Your training just kicks in, and you do something. And to your utter astonishment, you were able to stop him from doing the worst thing to you. It's more progress than you've made all this time trying to think harder and harder. Something just "clicked", and you did the right thing without thinking about it.
That's acting without thinking about it. You're calming your conscious mind down enough for you to allow your subconscious mind to take over.
Your subconscious mind is strange. It's vast. It's not a mind as such. It's more like a bunch of systems of parallel neural networks all operating at the same time and which figure stuff out without you consciously knowing it. It often knows the right answer way before your conscious mind has figured it out. Your conscious mind is totally unaware of most of it. Which is why sometimes you just have to stop thinking and allow yourself to be controlled by the subconscious, instead. You have to silence your mind, because the subconscious whispers.
Sorta. I'm not an expert in brain science. But that's as good as I can explain it. Take that with a grain of salt.
In Japanese martial arts, the concept of mushin is used to explain how you know when someone is creeping up on you, or there's something strange about the room you walked into. It's a weird feeling you're getting from your subconscious mind. It's saying this situation is different, suspicious, or dangerous.
For example, if you enter a room that you've seen many times before, but there's a slight change in the way the air is moving around the room, you won't consciously notice it. It's not something you're consciously looking for. But your subconscious notices it, and it has already determined there's an object somewhere in the room that could be someone hiding. It is sending you a signal that the room is dangerous. Allowing yourself to listen to these subtle signals can save you from being ambushed.
A lot of the times you get these signals, it will turn out to be nothing. There are many false alarms. Your subconscious isn't perfect. It's just noticing something strange. What that strange thing is could be benign or dangerous. Like in the previous example, was that "object" that changed how the air moved in the room a hidden person or maybe just a change in the position of one of the sofas?
We would be wise to develop our ability to silence the mind and listen to these subtle signals coming from the subconscious. In my experience, when I have let my subconscious take over, it has led to very surprising breakthroughs. When you experience it for the first time, it feels almost spiritual.
Hope that helps.