The black piece is a pretty powerfull magnet. So when I was filling some pin holes on the cowl I would place the copper pad on the oppisite side that I would be welding. Turn the red knob to tighten the copper pad against the sheet metal. Then weld the hold shut. If you can keep the copper pad flat against the metal I had zero burn through issues.
It even worked good on a few 3/8" holes that I got a little crazy with the spot weld cutter. Put a spot weld cutter in a drill, used a scrape piece of metal and made a couple little plugs. Then I used this thing told hold it together while I welded the plug in.