Second paragraph of third chapter:

The key maintains a direct link with the TARDIS at all times. Even if the TARDIS is isolated by a temporal disturbance the key should be able to summon it back (see Case Study, page 32). Conversely, the TARDIS can be concealed, a second out of sync with your current time zone, using the key as an anchor.

This is one of those really lovely BBC spinoff books, looking in detail at the TARDIS, presented as an operator’s manual and pulling together all the TARDIS lore from the first 55 years of Doctor Who (it features Jodie Whittaker but not any of her stories). None of this is new, but it’s put together very imaginatively and entertainingly. I had forgotten that many of the early stories include a TARDIS malfunction of some kind, and the authors heroically retcon everything together, even the Eye of Harmony from the TV movie. A lovely effort. You can get it here.