If the echo is incorrect she repeats the word until it is echoed correctly, or spells it aloud, with each letter being echoed. Obviously such an interaction can only succeed if the listener has the necessary skills and perseverance. The whole system depends on feedback - speak, repeat, confirm. Rachel and her listener must have similar levels of literacy, and the listener has to know Rachel's clarification strategies. Most importantly, both parties have to be highly motivated and prepared to expend considerable time and energy to enable communication to take place.
Similar feedback is needed when people are using communication boards.
As a communication board produces no printed or spoken output, board users have no way of knowing how their messages are being received unless their partners tell them. Another reason for repeating the message as it's created is that it keeps it fresh in the minds of sender and receiver. A message which is created slowly, over a period of minutes, places unusual demands on the memories of both parties. Speakers may forget what they're saying in a sentence taking no more than a few seconds. Imagine keeping track for the 5 or 10 minutes some people take to spell a sentence.