In principle it's learning to eat healthily... it's about eating the right kind of carbs and fats etc. There are three Phases, in Phase 1, which lasts two weeks, you cut out all carbs, expected weight loss for the first two weeks is 7-13pounds. The actual purpose of this phase is to fix your blood chemicals (cholesterol, trigylcerides, etc.).
In Phase 2 you start easing the RIGHT carbs back in, limiting yourself to a couple of servings a week. (The "right" carbs are granary bread, wholewheat pasta, brown rice, sweet potatoes - not real ones). You stay in Phase 2 for the duration of your weight loss, you can go back to Phase 1 for a boost, but it's advised you don't stay in Phase 1 for a long time because it's not healthy - your body does need carbs.
Phase 3 is the maintenance stage, its more liberal than Phase 2, but based very much on the same principles. In theory, that lasts for the rest of your life.
Co-incidently this diet was not designed to be a weight loss diet, it was created by a cardiologist to help heart disease patients. It is based loosely on the GI Diet, and the book talks a lot about insulin levels and stuff.
It's good, I like it and its relatively easy to follow if you know what you're doing. I lost almost 2 stone on this diet about three months ago, then I had my final exams at uni at the same time as working and then I moved to France.... I fell off the wagon. But I just jumped back on.
~Silence~