I saw one demo-ed at a car show about 15 years ago. one of the guy's watching said let me try that, He didn't have any sucess. so I walked on by, and didn't get interested until a couple years ago reading on Metalmeet forums on how to build one. It takes some technique. it works similar to using a torch to heat up a spot, hammer it down on dolly then cool with a wet rag to shrink it down.. The spinning disc against the body creates heat and the high spot expands up, you apply pressure to push it down , similat to using a hammer then lift and let the air swirling from the disc cool that spot to shrink it. and use a wet rag to take out the residual heat that spreads from the hot spot. It takes some technique to know how long to hold in one spot to create the heat, how hard to apply pressure and then to spin cool it.