I have watched it being done. Pretty fascinating. The two blocks are steel? rectangles with a semicircular groove exactly the diameter of the tube to be repaired along the long flat. They are placed on the frame so they envelope it almost completely, then are slid along it over the dents. I forget if it was done with just strong hands clamping the blocks or if more force was used. The tube is essentially 'massaged" with these blocks. Apparently the dent forces the round tube cross-section to a non-round shape. With enough coaxing back to round, the dent can no longer exist. It seems that a small dent is just another stable arrangement of the tube; that if plastic failure has not happened, the tube can be milked back to its original shape, sort of like a tacoed wheel can often be nursed back to true.

I had a frame with a TT dent that I gave to my frame builder to go to the painter to be painted in the same batch as the custom he was building for me. It came back with no hint of the dent. Now, that dent wasn't minor and the painter located the name directly over it, perhaps because a trace of it was still there. But I have never been able to see anything.

No expert here, but I am sure the paint will take a beating, It may stay intact, but it is hard to imagine that visible scrapes won't happen. But the bike should be as good as new after the dent is taken out. (And will be even if you do nothing with the dent beyond training yourself not to look at it. I raced my Fuji Pro with a real dent in the down tube after getting hit by a car. Made absolutely zero difference.

Ben