Something else to remember, is to work the dent out in the reverse direction from which it was created. This mostly applies to dents that are created from other objects like fence posts, cars, etc. I watched a bodyshop friend of mine once, and he was using a tool like the one you're looking at. He explained that dents stretch the metal so you have to work it out backwards from the way it happens. Otherwise you will spend way too much time on each dent.
