The Fast Return Switch is a component
of the TARDIS
from the series Doctor Who
. It is a quick and painless way to make the ship backtrack
its path through time
without all the messing about with calculations. Even Time Lords
get tired of doing maths.
The Fast Return Switch is basically a switch on the TARDIS console with the words "Fast Return Switch" written above it (in felt pen in the Doctors TARDIS). It is unclear whether the Doctor invented it himself or wether it is a normal device, as the events of The Edge of Destruction show it can be gravely dangerous if it malfunctions.
In its first and only appearance the switch jams and sends the ship back in time. I presume the normal function makes the TARDIS travel back along its own personal timeline, but in this case the spring under the switch sticks and the ship travels back in time towards Event One.
The idea that the Doctor built the device comes from the fact that he used a spring to work the mechanism, a primitive yet easily obtained system in the Earth of the 1960's. The fact that it went wrong so spectacularly means that it was not made by someone with a bent towards TARDIS engineering - something the first Doctor was not too good at, as he could never quite get it to work properly.
To date, and the best of my knowledge, the Fast Return Switch has only been used once, in one story. I assume the Doctor disconnected the system afterwards. It's probably for the best....