A further clue to what's described is in an earlier sentence:
‘These drills run at a tremendous speed,’ the father
said, ‘so when I switch on the drill the mileage
numbers on the speedo spin backwards at a fantastic
rate. I can knock fifty thousand miles off the clock in a
few minutes with my high-speed electric drill. And by
the time I’ve finished, the car’s only done ten thousand
and it’s ready for sale. “She’s almost new,” I say to the
customer.
The father thinks he's the first person to have come up with the idea—at least with a drill. (It's not clear if it's the use of the drill that he thinks was originally his or the idea in general.)
Turning back the odometer on a car makes it seem as it if has had less mileage, and, therefore, is in better condition than it actually is. People will pay more for a car with less mileage.