Thanks! Don't feel bad - I had to ask that same question on AK about a month ago. It's used when servicing equipment (stereo receivers, in my case) that might have the potential to do internal damage if things like final output transistors or heavier current-handling devices are shorted. If the bulb(s) stay bright for longer than a maybe a second, you know there's a heavy current draw plugged into the tester (and there shouldn't be) and you can power down - hopefully before any damage is done. All three bulbs are 60 watts, so you start off with just one bulb switched on, then two, then all three. This gives you ability to apply power gradually, while monitoring for bulb brightness. The 60-watt bulbs were just a guess, but they should work okay for a fairly-wide range of receivers. If someone wants to jump in with clarifications, please do so - I'm still kind of a newbie at this!