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The letters are in specific spots inside the case. Because the case layout is a standard, someone very skilled with the standard could read the word being typeset because they could see which spots were being pulled from for different words... since the spot in the case is the letter... you can know the letters of the word. This might assume that the letters were being pulled in order (or reasonably ordered).

it's like if you watched someone typing on a qwerty keyboard with no letters printed on it but you could still make out the words because you know where the keys are.

You could watch someone work and know what words they were putting down by where they were reaching, even if you were too far away to make out the letters themselves.

If you pick the letters strictly in order, it spells out the words. If you pick the letters by word, you can guess what word as there are only so many words. It'd be like watching a touch typist on a unlabeled keyboard for another example.