Wet polishing is a method of polishing a car which has gone out of fashion, but was still fairly common in the trade up to twenty years ago. The car was polished with a rotary polisher, fairly coarse compounds and kept wet.
The water would act as a lubricant and keep the panel cool. The abrasives in the compound would wear down, getting finer and finer as you worked, but wouldn't bind.
With the rotary polisher turned up to full speed, this was a very quick method of polishing a car and yet had little risk of causing burns. It was particularly effective on cars/vans with solid base paintwork which had oxidized. These older cars had a lot of paint to work with and a lot of the oxidized paint would be coming off, keeping it wet washed away residue.
This method of polishing has fallen out of favour, as has solid base paintwork, rotary polishers and compounds which were far less sophisticated than those used today.