I think the fuel cut off switch you have been reading about is a switch which is placed in series with the fuel pump supply wire sometimes also called an Inertia switch. Inside these switches is a mechanism which breaks the circuit if the car suffers a heavy bump, cutting the fuel supply to the engine and avoiding a potentially hazardous situation.
These switches can be reset by pressing a built in button. They should be rigidly mounted to the body in a suitable place so any impact energy is transferred to the switch.
Do a search on Ebay "Fuel cut off switch" you will find dozens of them, You also see the the square red rubber topped ones under the bonnets of many cars in the scrap yard.
The manual switch and relay mentioned in the last post is more useful as an anti theft device.
Edited by tommy13, 12 April 2010 - 09:07 PM.