The first thing that comes to my mind is a rolling mill, that is, stretched metal and nice little oil cans where the ding was. I'd like to here the thoughts from some of the real bodymen out there.
"Oil-canning" isn't something you're likely to run into with vehicles like our old trucks except places like the roof. You need a pretty large expanse of relatively flat/thin sheetmetal to make an oil can.
I'm not quite sure what this tool is they're referring to but it sounds like one of those things that looks like an old Ice block lifter with a large washer-like thing on one part and a body pick on the other that hits directly in the center of the hole in the "washer". You position the washer thing right over the dent and start tapping the pick end to push the dent out, but then you have to metal finish.
It's just a contraption to aid the bump man in hitting the exact spot with the pick. It's near impossible to do that with a pick hammer without missing the exact spot a couple times.
There may be modern variations on this tool that I'm not aware of but the principal is the same... unless I'm completely missing the idea.
