Originally Posted by swamper06
It shuts off your fuel pump in the event of an accident.
adding to the above
There is a sensor located somewhere in the truck-not sure where or if its in the switch itself-but it senses a sudden jar(ie getting hit or hitting real hard or flipping the truck) and it pops the switch like a circuit breaker shutting off the fuel pump so the truck dies and prevents the truck from running-possibly moving and causing more damage, and to make sure fuel doesnt go all over place to prevent fire/explosion
If you look under the passenger side of the dashboard tucked up under it on the firewall you will see a switch-its a pushbutton-this is the indertia switch(red button-black housing i think) for if you ever need it-push the button and the truck is running again-may have to turn the key on/off a few times to cycle the pump to get enough fuel to start but it should start again-only time this should trip is in the event of a severe accident