One of the most controversial topics in the fitness industry is "crash diets"--plans that are designed to make you lose weight really quickly. Some of these regimens can cause you to lose 20 pounds in under a month. Below is a testimony of someone who has tried this method.
There are a few things you'll notice about these kinds of plans:
First and foremost, they are very strict. There is no way to "cheat" or "splurge" if you are trying to lose weight rapidly. This may mean that you have to drastically change what you eat and avoid all types of junk foods (which tend to be high in calories). But this can work to your advantage as long as you don't go crazy once the diet is over with. It could be that you'll learn self-control from avoiding all the foods that put the weight on.
Secondly, there tends to be a pretty significant calorie restriction involved. A mores slow-paced diet may just require you to eat slightly below maintenance level calories. But dieting this way may require you to go down to as low as 60% of your maintenance levels. This cannot be done indefinitely, of course, but it can be done for a short time period in order to help you lose weight fast.
Last (but not least), crash diets are temporary. You simply cannot go on such severe calorie and diet restrictions for over a few weeks. Even if you could handle it physically, psychologically it would take its toll. You would be miserable as well as those around you.
Here's a pretty good summary of one a popular plan called the 3 week diet: