opto kill can function as both. you can set radio to take eng servo to fail safe, signal loss will kill engine, and you can also use the throttle cut feature on your radio. It took the loss of a finger at our club to put better safety measures in place. I like the safety restraints but there is no better proceedure than having an assistant to assist with a preflight inspection and to insure the aircraft is being restrained during all startups, and that any loose eguiptment in the immediate area doesn't become a flying projectile.