It is just your ordinary mind doing what it does when you have stopped monitoring what your mind is doing and are just doing it.
If you really enjoy doing anything and you find you have utterly lost track of time in doing it, you dwelt in mushin at some point. Good books, skiing, driving -- anything really. It is quite easy with things that are engaging and pleasant. It is really much more straightforward and less mystical than people who are selling something may make it out to be.
The difference in budo is in unlearning one's inhibitions about physical threats and space. You have to become as absorbed in the immediacy and the process of being attacked as a painter becomes in painting a detailed landscape for hours on end. That is not so easy -- because it involves completely inverting one's sense of threat, about which some people have very deep inhibitions that they are often unable to remove without grave difficulty. Mushin is dropping into the same groove routinely under physical threat -- which occurs only after much practice, and for which there is no substitute.
As O Sensei said -- it is simply adopting an attitude of love, or, if you prefer, as Jesus said -- loving one's enemies as oneself and turning (tenkan) the other cheek (irimi). The hard part is not knowing what it is or what it feels like (I daresay have never met anyone who has not experienced mushin in some setting) -- but being there while under hard circumstances. A hard slog up a mountain is just one damn blistered foot after the other -- but it ceases to be a such problem when we cease to distinguish the movement of our feet from the rest of our body, and -- as Peter O'Toole said in Lawrence of Arabia -- "The trick is -- not minding that it hurts."
Mushin is like this.
Thank you for the excellent post, Mr. Mead
The quote on my profile sums it up for me. In-adversity, in-joy, in all things......."this is this."
There are people saying that if you're asking, you aren't in it. Maybe that's true; but it is in you
and the fact that you are asking this question is a clue that your timeless self is speaking.
O'Sensei spoke of aikido as 'the way of the mountain echo path'. Like a call out to a mountain across a great divide your question will be answered and magnified by the power of 'the echo'. Your call will be sent back to you and you will likely smile at the sound ( mu-su).
Enjoy your journey! You're on your way.