Like I said, few people can achieve this kind of weight loss, not because its because its impossible but because it is not the "quick fix" that they desire. Set point tells your body what it should weigh. When you try to lose weight with a traditional crash diet, it sends your body into emergency mode. This set weight is what you should weigh for survival, and it will fight to keep you there. As far as your body is concerned, this if for your own good.
The theory states everyone's body has a genetically determined range of weight and temperature that their body will try to maintain to stay at optimal health. For example if Bob has a weight set point of 75 kg, his body will try to stay around that weight. If he eats less and exercises more, his body's metabolism will slow down. If Bob eats more and exercises less, his body's metabolism will speed up. The body will attempt to compensate for Bob's eating habits in order to stay in that range. So by eating more and ensuring you don't over exercise (bootcamp, zumba, body pump etc) then you can speed up your metabolism and eventually lower your set point.