CrazyJ23 is correct in his statement that glycogen levels do not need to be fully depleted in order for the body to rely on fatty acid metabolism. Simply consider how many competitive bodybuilders (non-enhanced more so than PED assisted) rely on considerable levels of carbohydrate intake each take during contest preps.
Obviously if your body can store an average of ~400g of glycogen at a given time (there is some disagreement on this # so just go with me on this), and you’re ingesting 250g of carbs on an average day while dieting, it is highly doubtful that any normal person juggling a job, school, daily social interactions, and even weight work and cardio, will be able to make use of 650g of carbs/glycogen in time to still “burn some fat” each day.
The use of refeeds, high days, however someone may choose to call a bump in carbohydrate focused #s, does have an effect on hormonal levels in the body, but I view this not as something that raises levels of fat metabolism, but simply more of a preventative effect. As your hormonal profiles get ‘worse’ (to refer to the process in black and white terms) through extended dieting, the body wants to rid itself of anything that is metabolically costly, and thus the prime focus is simply on self preservation.
It is this constant overriding drive that leads people to state that there is always some muscle loss during a strict diet. Not that I agree with such an absolute statement, but the body definitely wants to. It is the use of an above typical influx of carbohydrates that can have a positive influence on specific hormone levels in the body that sort of wards off the negative issues that are building. Sort of keeping the catabolic wolves at bay so to speak.
Dr Klemszewski always wrote that at any given moment, both carbohydrate and fatty acid metabolism are occurring in the body. It is simply the ratios of how much one may be contributing at any time that we can affect via specific activities or dietary choices.
(I’m sure I’ve simplified matters considerably compared to what you might find in a science text -lol)