Ever heard of this?

[RoadRaven]

My LBS (Hi Chris!) says "The device she speaks of was called a " Tire saver " mounted to the brake caliper using a spring piece of wire (generally made from a bicycle spoke) to constantly brush the tire tread in hope of dislodging glass slivers and the like before they could penetrate all the way to the inner tube."

Ok... so they brush the tire... constantly, to dislodge glass etc...

Then how does this work >

But they work, and don't drag on the tire.

If they brush the tire, they must cause some drag on the tire... yes?