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 type

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).