Ground Switch

It's found on amps that have a two-prong plug that's not polarized. With such a system, one side of the line is "hot," and the other is "neutral" (near ground potential.) With a nonpolarized plug, it's a random chance which is which when you plug in. The "ground" switch connects a capacitor that's connected at one end to the chassis to either conductor in the power cord, or to neither. If it's connected to the "hot" one, typically you'll get more hum, and since capacitors conduct alternating current, you can get a little tingle out of touching something connected to the chassis (like your guitar) and something else grounded. If the capacitor shorts, you have a 50% chance that your amp chassis (and guitar) will be "hot," directly connected to the power line. That's why that capacitor is commonly called "the death cap." It's best to convert amps wired like this to a three-wire power cord with the chassis directly grounded to the third wire.