When your disc brake rotor gets all wombly-pombly on ya, you have to straighten it out. A very light pressure is required. I use a piece of white paper that I hold behind the brake as I sight down the rotor, so when I’m looking down thru the brake, I can see which side it’s rubbing on. Clean the tool before using with denatured alcohol, so you don’t get old oil on the rotor and have a sad day.
Unior makes this tool in the Czech Republic, home of the beer emergency hut.