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")
After the Master tricked the Doctor into giving him the TARDIS key and deceived Ian and Barbara into leading him to the ship, Ian and Barbara were able to overpower the Master as he attempted to steer the modified TARDIS, then used the fast return switch to take the TARDIS back to Destination and reunite with the Doctor and Susan, albeit a couple of years after their departure due to the Master's temporal manipulation experiments. (AUDIO: The Destination Wars)
The Doctor once used the fast return switch to return a young boy named Robert to 1965, exploiting the fact that the TARDIS's last few trips had occurred while the ship was linked to the boy's own time machine. (PROSE: The Little Drummer Boy)
In 1483, Peri Brown 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 used the fast return switch to return to 305 and rescue the Eighth Doctor after he was lost in the past intercepting the Nimon pod on its way to Earth; this return trip also resulted in the Doctor's foe Sebastian Grayle being killed by his own past self, who was disgusted and horrified at what he had become in that future. (AUDIO: Seasons of Fear)
When the Eleventh Doctor fought the Shroud, Clara Oswald used the fast return switch to draw the Shroud to Venofax, the last planet the TARDIS had visited, where the planet's avocado-tainted sea significantly weakened the Shroud. (PROSE: Shroud of Sorrow)