The idea behind this diet is that you eat very little then eat whatever you like on alternate days. The days you eat very little are almost like fasting, but the diet recommends you still eat some low calorie but nutritious foods (and supplements) to avoid health problems.

This diet works on a few levels. First, calorie-wise, you'll end up eating less overall for the week. Second, the shifting back and forth between semi-starving and eating normally puts your body's metabolism into a kind of semi-panicked state where it wont store calories as fat (well, hopefully). Also, because you do get to eat what you want, the psychology of it helps prevent feeling deprived. You can always say to yourself, "Oh, I'll eat that tomorrow."

Clearly, it's also important to not actually pig out majorly on the days when you "eat what you want". Or, you could actually end up having more calories for the week than you did before the diet. The diet stresses eating healthy, exercising, etc. like most diets.

Another nice thing about this diet is that it's not that complicated. Once you get down what you need to eat on fasting days, it doesn't take much thought or effort. It's a calorie-reduced diet without much calorie-counting.