I could not figure out how these guys do their magic until I watched this you tube video, I'm sure the author won't mind me grabbing a couple of screenshots. As a noob I can't post links so search on YouTube for 

M3 BMW PDR Dent Repair Training Tutorial / Paintless Dent Removal

and you should find it.

As a teaser, here is the dent he is working on, in a very difficult place on a fold in the body panel before and after (dent is arrowed). As the narrator says, "It's a crease inside of a dent, that goes across a body line - the body line is completely crushed" :

A little clue how they get to such a great result :

The process is fascinating to watch, YouTube will show you plenty of other videos showing the entire process.  I'd say you need a fair bit of skill to work at this level.