Hot scraping is just scraping while the wax is still warm. Don't overthink it.
As for what it does, it removes more wax than is normally removed when you scrape a cold board. If you ride mountains with sub-par conditions there is typically a lot of dirt or other pollutants. When you wax your board you typically break these pollutants free from the base and they mix with your wax (the reason the wax looks dirty when your spreading it sometimes.)
Hot scraping is used to remove these pollutants as an alternative to using a base cleaner that strips all of the wax out of the base. The only time you ever need to strip all of the wax out is if you are doing p-tex repair work. Other than that, keep layering wax on top of wax, on top of wax, on top of wax. There is no such thing as overwaxing, it can just get a little expensive if you go crazy.