You change the area that is being shaded by the tool by moving it closer or farther away from the paper. The tool must be kept in constant motion so it doesn't create a defined shadow on the print. Some experienced printers may use just their hand as a tool. You can also make your own tool out of cardboard to fit a certain area in a print. The reverse of dodging is burning where a hole in a piece of cardboard is used to give a section of a print more exposure. It's best you read a book on printing for more information. An older but good one is by Lootens "On Photographic Enlarging and Print Quality" but there are many others.