An RTC unit (or Reverse Tachyon-Chronon unit) was used to control reverse tachyon-chronons.
When placed in an ex-area, a kind of transcendental field, an RTC unit would create an almost impenetrable barrier. The more RTC units that were kept together, the more dangerous they would be.
The Euterpians disguised them as books, the pages coated with RTCs. Flipping the pages moved time forwards or backwards for the user (similar to The Worshipful and Ancient Law of Gallifrey (TV: Shada)). If the book was put under an x-ray it would heat up; under a spectrograph, the feedback would destroy the machine. The pages couldn't even be folded or bent as they would simply repair themselves.