Do the FOBs (sensors) only work by distance?
No, they work based on both distance and by submersion in a body of water. If you fall off your boat into the water it will be triggered when the sensor is submerged because communication between the host and the sensor has been lost. Please note: the sensor can be splashed by a wave and will not trigger, it must be submerged in a large body of water to activate the kill switch and shut off the motor.