Not sure exactly, but it is similar to sonic welding on plastic and aluminum. Basically, you have a computer control device that creates a resonance in a solid piece of steel, called a horn. This piece is tuned to create the best resonance for a certain material at a certain hz. The horn is touched to the piece, and if it is set up to weld it, the vibrations heat up the material enough to cause fusion. If it is set up to remove material, the resonance is set to affect the stud or piece that needs to be removed, and not the surrounding material, as every material has a different hz that it is affected by. The horn is touched to the material, and the material disintegrates. It's a pretty cool process, but I wouldn't recommend being in the same room while it's going on- not loud, just a REALLY annoying sound.
Jared