I like to refer to the disc as a "Heat block" it heats only the high spots and you cool them to shrink them. Like I said I am no pro but when I was using on every day when I first got mine I was blown away at what could be done with it. Wish I had photos, one Toyota quarter panel I did was absolutely amazing. The metal was folded over it's self at the rear by a glancing blow with something very solid. The damage was about 18" long with this corner being folded over with about an 1/8" radius fold hanging off the quarter under the tail light. I got it all back into shape with only a few coats of primer, it was WILD.
I got mine from Ken at Sun Chaser tools and bought his 5000 RPM grinder and was never sorry. I would use it for collision work on big bondo jobs like the bed side of a truck or something just to get it tight for bondo. It worked great for that, I could of repaired it with no bondo but being there just ins't time, I would use the disc to tighten up the oil caning panel for filler.
Here is a review I did on a DVD I got from John Kelly, it doesn't appear he is in business anymore but the into on the disc still stands. Ghia Specialties metal working video review.