The previous poster was not talking about installing a fan, but replacing the speed switch. Yes there is only one wire to supply power from the house to the fan. But in many fans, there is a speed switch that has one wire that bring electricity in and three that go out. They go to three different windings (coils) in the fan motor itself.
If you are checking VOLTAGE, the number of AMPS running through your meter is close to zero. Yes, I have blown the fuse in my meter once, but it was because I got frustrated and accidently went to check a live circuit with the meter set on resistance (OHMS). It stupidity, not price of the meter, that was at fault.
It makes perfect sense. He is talking about the house receptacle the computer was plugged into. AC current works by one side doing a push/pull cycle of electrons while the other side is grounded. As long a electricty is flowing first one way and then the other at the proper rate, it usually doesnt matter which side is the live one. I knew (not well) a guy who wired in some lights for some illegal plants he was growing ih the basement. He bypassed the electric company's meter (and in the process the breaker panel) to save money. He later went to change a burnt out bulb while standing on a damp cement floor in bare feet. If he hadn't switched the wires, he would be alive today. He had the screw threads of the bulb socket live instead of the little dot in the middle.