A fast return switch was a component of a TARDIS that returned the vessel either back through time or to a previous location in space and time.
After leaving Skaro, the First Doctor used his TARDIS' fast return switch in an attempt to return Ian Chesterton and Barbara Wright to 1963. However, the spring in the switch became stuck. This would have caused the ship to go back in time to the creation of a solar system. As this would have resulted in its destruction, the TARDIS crashed itself and attempted to warn its crew of the danger, eventually succeeding. (TV: "The Brink of Disaster")
In 1483, Peri and Erimem, who were stranded in that year, received a letter from the Fifth Doctor advising them to use the fast return switch to get back to 1485. When they did so, the TARDIS disappeared. (AUDIO: The Kingmaker)
Charley Pollard used the fast return switch to return to 305 and rescue the Eighth Doctor. (AUDIO: Seasons of Fear) Later, Charley used the fast return switch to fly the TARDIS back into the path of time torpedoes. (AUDIO: Neverland)
When the Eleventh Doctor fought the Shroud, Clara Oswald used the fast return switch to draw the Shroud to Venofax, where the planet's avocado-tainted sea significantly weakened the Shroud. (PROSE: Shroud of Sorrow)