The emergency unit was a part of the TARDIS which, according to the Second Doctor, "move[d] the TARDIS out of the time-space dimension — out of reality". When the fluid links failed after having been apparently overwhelmed by the lava encasing the TARDIS on the planet Dulkis, the Doctor reluctantly used the device. The TARDIS was instantly removed from time and space and all the TARDIS' instrumentation went dead.
The emergency unit had a timer on it. Because it was considered dangerous to be outside reality, the unit would automatically pop its users back into normal space after only a few minutes. (TV: The Mind Robber [+]Peter Ling, Doctor Who season 6 (BBC1, 1968).)
The Fourth Doctor used it to escape the clutches of the Black Guardian, who wanted him to release the Darkness upon the universe. (PROSE: The Well-Mannered War [+]Gareth Roberts, Virgin Missing Adventures (Virgin Books, 1997).)