At the beginning of your diet, when you lower your calorie
intake and increase your exercise calorie burn, it is a "shock" to
your system. Your body responds to this by burning fat and thus,
you lose weight.
However, as time goes on, your body adapts to the "shock" and
makes adjustments (such as lowering your metabolism), that stop
your weight loss entirely.
This is why MOST diets fail!
The recent "high-tech" approach to solving this problem and
producing a diet plan that causes you to lose weight for a LONG
time, is what is called "calorie shifting".