While carb-cycling will not help you lose more fat than eating a baseline level of carbs throughout the entire week, some people prefer to eat more carbs on training days.
By using the high/low day feature on your Avatar profile, you can increase your carb intake on certain days and will also be able to control the magnitude of the increase. Keep in mind that as you increase your carbs, fat will also increase slightly.
If you have a high or low day that you also want to adjust the carb/fat ratio on, use the Edit Balance tool in your tracker to make that adjustment for the current day.