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I've seen this kind of thing before -- if it's what I'm thinking of it uses several LEDs controlled by a computer that flashes the message like one of those moving marquees you find in lobbys.
Except that instead of a big grid of LEDs, it just uses a row or two but flashes in the same way.
When you wave it through the air, your persistance of vision lets you see a message almost like as if it were one of those marquees.
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