it's like a very thin slide rule that measures the gap between the 2 electrodes of the sparkplug. Honda has a certain measured gap that is needed for the sparkplug, I dunno what the gap is supposed to be.

And the plugs have to be tightened to a certain torque measurement.. which is in lb-ft. Honda has it set at 13lb-ft, turning the wrench when it reads 13lb-ft. ppl who dont use it, tighten with wrench and once it stops turning add another 1/4 or 1/3 turn. But's that's just feeling, mainthing is not to tighten it to the point where you start strippin thread...

Hope that helps, I'll be trying it tomorrow..