Its actually pretty simple but requires a complete teardown of the car and a lot of prep. You cut a 1.5 mm slot along each seam, inside or outside of the spot welds and deep enough to only cut through 1 of the panels spot welded together.
Then, using a TIG welder (no MIG too much heat - panel distortion and discoloration) you run a bead in the slot joining the the two panels back together. Then you carefully grind and fill everything so its smooth and then undercoat or paint.
Again, its hundreds of hours of prep and clean up after. So, whichever racing series you're cheating at better pay a lot in prize money. I also suspect that, since I was told of this trick, the cheaters have found another, easier, better way of achieving the same thing. Whenever you're told of a "new" trick, you can bet that its not the "new, new" trick.