A safety rotor is used when the normal rotor stops spinning than suddenly the safety rotor starts spinning a camera can be on the normal propeller and if it detects that the propeller stops then suddenly the safety propellor starts spinning this can also be connected to a  ai computer that detect that the normal propeller stops spinning than the computer starts the safety propeller this can be used in flying cars that fly with propellers also when you have two propellors on your shoulders you must have two safety propellers on each shoulder one I ever invented a flying egg it's kind of a helicopter but it has a safety propeller it has also o jet engine and if those start then a rear propeller will start spinning and goes in the opposite direction of the jetes so the act will be kinda Pulled Apart but in that motion that will be a lot of Steering possible more control in curves i mean.