I used/made the flippers when I was a trainee panel beater, there was also a shrinking hammer (not literally) which was used in the same way, some times we used to make a hot spot with the blow torch and then beat the panel towards the hot spot, this was not only to use less filler, but it would strengthen the panel by removing some of the flex caused by the crash. On smaller dents one the panel had been shrunk down a body file could be used and some times this could be done as a metal finish. If the were slight discrepancies on a high end high value car lead would be used as the filler and body filed down rather than using plastic filler.
On large panels like a roof with numerous small dents it was possible to bring the dents up by heating with a small flame aimed directly at the centre of the dent and immediately using the body file over the top, I did 30 dents in a Triumph Aclaim roof this way and no filler was used at all in the repair. I'm really not sure just how that machine would shrink the panel and at least with hand tools there is a lot more control.
John