To say that 無心 (mu-shin) translates to "no mind" is sort of like saying that 道 (dao) translates to "path." 道 implies expression, and traveling, and knowing, and experiencing "path" is just the shortest English word that might approximate the idea. The same thing happens with 無心.
Most of the trouble is because of 無. If you read the node on 無 carefully, you have a tiny idea of what 無 is. I know what 無 is, but I still only have a tiny idea of what 無 is. It's nothing, a void but a nothing so vast it's bigger than anything ever has been or will be. 無 is so empty that the words "empty" and "void" aren't adequate, because they imply a lack of space, when 無 has nothing, not even a lack of space. It's the mass of a point with no length, width, or depth you don't think it's there, but you can think about it, so it must exist somewhere. 無 can be defined by any word, after you repeat it to yourself enough times so that it loses all meaning.
Such a concept doesn't have a word in English, so it's sometimes translated into "no" or "not."
Then again, it's often left the way it is: "Has a dog the Buddha nature?" "Mu!"
心 is where you can find the habit to cling to things in every human. When a loved one dies, 心 pains you. When you discover a treasure from your past that you had thought was gone, 心 rejoices. 心 is not simply your "mind" or "heart".
A state of 無心 can be very important in martial arts. To willingly adapt this emptiness-of-mindset and then physically fight, letting the body be shaped by evolution and the laws of physics and the unconscious power of the involuntary brain, is not an easy thing to learn or master. You become unaware of your attire, or or your surroundings, or what you opponent looks like — but you can feel your opponent, you almost taste the next move, you sense your opponents' emotions somewhere behind your eyeballs. Any injuries previously sustained stop their whining, the pain sits patiently and waits for you to finish what you're doing, as your body is obviously too busy to be bothered with pain. It wouldn't do any good anyway, as your 心 isn't connected to the rest of the body along the routes that pain is used to traveling.
You can't be taught how to tell if someone is in 無心 state, but you can feel it.
A state of 無心 can be achieved while doing anything, but it seems easier to discover when doing something that you repeat many times. Typing, exercising, running, playing music, noding...
無心 should not convey a negative emotion. It feels calm: detachedness, but a feeling that there's nothing to get worked up about. Time, that thing that lets you know change is happening, is no concern — the brain is unaware of change or any way to measure it, paying unconscious attention only to the task at hand.
Remaining level-headed. Acting in the moment, without needless thought. Temporarily setting aside the instinctive reactions of the body. Quieting the restless spirit inside you so you can ask it to do what it's yearned to do for years. All this and more is 無心.