Product review (5 stars): Works exactly as expected.
I was installing a Aprilaire 500 series humidifier on my furnace and the documentation on the wiring wasn't very clear as to which wires and terminals I should use.  Purchasing this solved the problem and I didn't have to worry about making a mistake and damaging something in the electronics on my furnace.

This relay simply attaches AROUND the existing wiring (Neutral or Common, probably white) going to the blower motor without making any electrical connections.  It senses when there is current flowing to the blower and closes the contacts of the wiring (attached yellow) to the humidifier.

Makes installing the humidifier very easy no matter what type of furnace setup that you have.