back in the day when power wasn't reliable nor was the install as to where the hot wire was, you could switch 'polarity' of the power. the 110v would run on the 'hot' (black usually) or nuetral (white) by a flip of the switch. what it does now since most 110v (120 or whatever depending on where you live, 117v here) is regulated and hot is (should be) on the black wire, if the switch is in the wrong position voltage is applied through the amp and into your guitar, straight through your stings to you. as you touch another path to ground (anything else plugged in) you get a nasty shock. a real zinger on the lips if you eat your mic.
since I haven't changed my 2 prong yet(I know, retard) I put a white dot on my 2 prong plug indicating nuetral. and my polarity switch down. test and play. I also use a power conditioner which tests and regulates the voltage going to my equipment.
my explaination of the polarity for these amps may be off, sorry, there are wiser individuals than me on this site. fact is polarity switch up, I get zapped. down I don't because I test where the nuetral goes and plug it in that way. easier safer solution, trade that plug and cord out for a 3 prong and I think disconect the polarity switch