She is referring to "krenkelkan" voice inversion encryption, in which all frequencies were rotated around 1KHz carrier. High frequencies became low, low frequencies became high, voice became unintelligible.
During WWII, the Germans actually trained listeners to copy voice inverted signals by ear. They called the encryption system "krenkelkan" which is what the word "telephone" sounds like when inverted.
It's fairly easy to undo. Aircraft Radio Corporation made some decoders which had a total of four tubes in the whole thing.
--scott