This checks the polarity of the 3 wires in a standard home. The bottom switch emulates reversed hot and neutral wires. Left switch opens neutral, right opens hot. Which ever bulbs flash, consider then as "on".
The bulbs from left to right are 1, 2, and 3. They indicate the following:
1 off, 2 on, 3 off -Open Ground
1 off, 2 off, 3 on -Open Neutral
1, 2, 3 all off - Open Hot
1 on, 2 off, 3 on -Hot/Ground reversed
1 on, 2 on, 3 off -Hot/Neutral reversed
1 off, 2 on, 3 on - OK
