Servo function 30 - motor enable switch (EngineRunEnable)

This function is described as giving a certain PWM when disarmed and another PWM when armed. It would be ideal for something Im trying to do, however it dosent seem to do anything. Its shown as available in copter and quadplane. I think it gets renamed for I.C engines.
Is there another setting somewhere that makes it function as described?
Thanks

Edit - I was thinking perhaps it needs ICE_ENABLE=1 which should be available in copter, however I dont see it in parameters.