Master Switch is available only on Digitrax Series 3 decoders. It allows you to program the Function 0 button to turn off all functions when Function 0 is turned off. The nice thing is that when you're done with the locomotive, you only have to turn Function 0 off to turn all functions off. The bad part is that if you're not done with the loco, turning the headlight off turns all other functions off whether you want to or not. If you never turn your headlight off until you're done with the loco, then this is definitely a good thing.

Had Digitrax made it available for any function other than Function 0, it would have been more versatile. But because you always use the white and yellow wires for front and rear lighting, and always want to use Function 0 to turn those lights on and off, function remapping doesn't even help to make it more versatile. If they had put it on Function 4, for example, you could have mapped it to any function you want (even Function 0), and not interfered with anything else.

Programming involves CVs 49 and 50. You actually have several options, but all are for Function 0. The right hex digit of the value controls what effect you want for Function 0. This can range anywhere from 0 (no effect) on up to any effect that is offered. It's the left digit that specifies the rest. The left digit can be 4 or 5 to implement the Master Switch without controlling ditch lights, or can be 6 or 7 for forward or reverse ditch-light control as well as Function 0 Master Switch.