Using the FURminator does take some practice. I have made Gio's butt nearly bald when I first used it. It was a situation where I was so amazed at how much hair I was getting out, that I just kept going ... and going ... and going. Poor Gio. But it grew back in, no problems. I go off of feel a lot when FURminatoring (new verb!). If the tool glides through easily, then keep going. As soon as it starts to feel choppy or rough, stop. You've got all the undercoat you are going to get.
I line brush when I use the FURminator. Meaning that I part the hair down to the skin, and work at the very base of the hair with the FURminator. Short strokes just to loosen the hair, then use a slicker brush to pull the hair out. I find that if I try to pull the FURminator right from the base of the hair to the tip that it ends up breaking some of the guard coat. By line brushing and just working the base of the hair, the guard hairs remain undamaged and you only remove the undercoat.
Another thing to keep in mind, is to not work on one area for too long. I only do about 5-7 strokes per "piece of skin", and not pressing down overly hard. The teeth of the FURminator are quite sharp (comparatively), and can damage the skin if you work one area too much.
When introducing the FURminator to your dog, I find it easiest to just do a small patch at a time. When I started, I did 1/4 of a dog per session, one session per day. As I got used to using the FURminator and the dogs began to accept it well, I would increase the area I would groom. Now I can do a whole dog in one session, usually with a break half way through to stretch my back and let the dog wander around for a minute.
I can completely strip the undercoat of both of my dogs in about an hour and a half. I try to do this about once a month in the summer time, slightly less often in the winter. In the summer, I try to get as much undercoat out as I can. In the winter, I use the FURminator sparingly, only thinning out areas, not removing the undercoat completely.
Here is a brief video of how I use the FURminator. Keep in mind that the boys were just stripped yesterday, so there isn't much undercoat for me to remove as a demo. I jumped around a lot trying to find a good place with some undercoat left. Normally I would move very systematically from the hindend of the dog to the shoulder.
And no, the FURminator people are not paying me. Though, if any FURminator people are reading this, I would be happy to accept "thank you" presents! Have your people call my people.