I saw one demo'ed at a car show several years a go, ,someone watching asked to try it and in a couple ofminutes didn't get it to work. the technique is similar to using a torch , hammer and dolly. heat up a spot, it expands, work the lump down, then let it cool and shrink. You kind of float over the surface apply enough pressure to heat up the high spot, enough pressure to force it down then move to another place to let it cool while you heat up the next spot. You usually don't want to put on enough pressure to get a large area blue.