Charlie, think of it this way, when the motor is on, and if it has a charging circuit, it pumps electricity back from the motor to the battery, the flow is continous, as long as the engine is turning it generates electricity. If you break the circuit, by switching it to off, you are taking out the recipient i.e., battery, the current has nowhere to go but go back and blow your rectifier....... this is how the Merc man explained to me.
Turning off ignition, your engine also dies, so no electricity being generated.