This article is from the The Monster Truck FAQ, by Jeff Hawkins with numerous contributions by others.
A. Let's call these by their official name, "Remote Ignition Interrupter"
or `RII'. This is a specialized radio receiver mounted in the truck
and a hand held transmitter tuned to the frequency of the RII. The
transmitter looks like any other `Walkie Talkie' but has the ability
to signal the Receiver with a special code!
The Remote Ignition Interrupter can be used to `interrupt' the
ignition by radio should the truck veer off the track and/or the
driver become incapacitated.
This `RII' attempts to shut off the engine but it doesn't stop or
"brake the truck".