It is often translated as "empty mind". Yes, it is written as "There is no mind" in Japanese.
It is controversial as people are asking whether it is possible to have no mind. Well, I do not know.
However, here is what I know and a bit of explanation of the empty mind.
Here is what empty mind is not:
These are not considered to be "empty mind". The mind is just gone somewhere and does not exist here on the earth.
Ok here is a good example.
When you drive a car, how do you drive?
As I also mentioned in "Enzan no Metsuke", you look everywhere to make sure that you and others are safe.
In other words, your mind flows everywhere it is necessary to be so that you will be able to react to any sudden incidents.
It is hard to drive if you think about it. You have to step on the accelerator and the brake and look at the side mirrors and rear mirror.
Besides, you must check the speedometer and pay attention to signs, people and other cars ALL around you.
How can you do that?
That is because you mind does not stay in one place such as just looking at the rear mirror not caring about the front or vice versa.
Your mind is working hard to make sure everything is ok.
Do not stop the flow of your mind at the one place.
In kendo, for example, do not be obsessed by winning or worried about losing. Stop thinking what your opponent is going to do.
Instead of thinking about a lot of stuff in your head, just go with the flow of your mind!
Mushin is not total emptiness of your mind. Your mind is there but working hard and it looks like it is empty. Let your mind do what it is supposed to do.
What don't we stop thinking about what mushin is but just let our mind go. We may get somewhere new! :)