They were controlled and utilised by RTC units which the Euterpians disguised 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)). It could affect a person's mind, making them recall memories and making things younger/newer. 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.
When placed in an ex-area, a kind of transcendental field, the RTC unit would create an almost impenetrable barrier. The more RTC units kept together, the more dangerous they would be. It would be better to keep the units away from each other.
The Time Lords used RTCs for a time before they were banned, being considered dangerous. (PROSE: Invasion of the Cat-People) Younger Time Lords, like Romana I, still knew of them. (PROSE: The Shadow of Weng-Chiang)