Here, this should help explain the optical part of these devices.

http://en.wikipedia.org/wiki/Opto-isolator

There are two types of optical kill switches in common use for gas engines. In the first type, all the electronics are contained on one board so the isolation is only the small distance that can be had on that board. In another type ... made by Smart-Fly, they actually use a piece of optical fiber between two separate boards so the only connection between the receiver and the ignition is the length of fiber. This is true isolation in my opinion. Single board units can never offer this degree of isolation.