Q: What purpose does carb cycling serve?
A: Carb cycling, or moving from a lower-carbohydrate intake to higher carbs, is a technique bodybuilders use to strip away bodyfat. Generally, a bodybuilder will eat 100—125 grams of carbs a day for 3-4 days followed by 400—500 grams on the fifth day. Then he'll return to low-carb days and repeat the process. This technique seems to keep the metabolism elevated while a constant lower-carb intake can actually cause the metabolism to slow. Another approach is simply to cut your carb portions in half for 3—4 days, then go back to eating a higher carb intake on the fifth day. After that, return to the modified lower-carb intake for another 3—4 days and repeat.
The key is to keep your body guessing, as opposed to letting it get accustomed to one particular nutrient intake day after day. Just as you need to change things up in the gym for continued gains in strength and size and to avoid plateaus, you mustn't let your body get too comfortable with the foods you eat. This is also why the occasional cheat day is considered productive for bodybuilders on strict diets—it's all about shocking the system to elicit a positive response.