In regards to how the Giant Scale racers are doing it there are two answers. First, on an engine with a magneto, the RC Switch is used to ground the coil to the case, instantly killing said motor due to lack of spark. Secondly, if the engine is running an electronic ignition, the RC Switch is placed in series with the positive or negitive wire going to the ignition battery.
Then again, if the racer is using glow fuel without ignition, you have to close off the butterfly valve.

Kai

PS I'm not familliar with the fiber optic switch you mention. Are you refering to an optically isolated switch?