It all has to do with glycogen, your body's stored form of energy. For every gram of stored glycogen, your muscle also stores 2.7 grams of water. So basically anytime you increase your carb intake, you can expect to hold more water, and if you cut carbs you can expect weight loss (water, not necessarily fat).Part of the reason why people fall in love with low-CHO diets is because they see a quick reduction in weight, but a lot of it is water-loss.
In the past I've followed diets that emphasized cutting carbs, and consequently looking flat, and then having big CH0-ups and then feeling huge. IMO they're overrated and the whole process is pretty dumb, except before a bodybuilding competetion.