Oh Jimsy... you must be so young!!
Back in the olden days, AC wiring only had two prong plugs in the wall instead of the three prongs used most places today. That third added prong being a ground wire, which if you have one on your amp is directly connected to the chassis.
Well, not only did we have only two prongs, both were the same size and you could fit the plug into the wall socket one of two ways. One way was "right" and the other way was "wrong".
What made it right or wrong was the assumption that was happening inside the amp - one side of the plug was "connected" to the amp's chassis through a capacitor, and hopefully that side was the side that was connected to ground.
If you got the connection wrong, at worst it could mean anything from a nasty shock to death.
But it also caused the amp to have more hum and noise than usual. So to get it right, you might have to flip the plug around in the socket and leave it the position that created the lowest amount of hum and noise from the amp. But that could be a pain going back and forth with the plug especially with a tube amp that needed to warm up, so some manufactures put in a switch to do that flipping the plug in the socket for you and much easier.
Sometimes this ground switch is called a polarity switch because it flips the "polarity" of the AC ground.
If your amp has a three prong AC supply cord, you probably won't have to worry about it as it may have been disconnected. But if you have a 2 prong cord, flip the switch until you get the least amount of noise out of the amp.
I don't recommend connecting both a 4 and 8 ohm cabinet.
