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I believe it loosely related to concentration.
I saw on 60 minutes about a machine that could tell if your brain was thinking of a hammer as supposed to a house, for example and it did a bunch of thing like thing of an orange or a dinosuar and the machine would tell you which one your thinking of and its accuracy was insane.
I can imagine that they scaled that machine down into just a little headset that just measures concentration. I don't think it would be perfect either, I'd love to try it though, just not for 80 bucks. I bet it breaks easily.