You are likely familiar with carb-loading or adjusting your diet to be more carbohydrate rich in the days leading up to an event.  The objective is hyper-saturating the muscles with energy storing glycogen, "topping off the tank" and maximizing energy stores. Some athletes are now experimenting with their diets by consuming mainly fat in a period of a few days to a week prior to carbo-loading. The objective is to train the body to utilize more fat as a fuel source and spare the precious (and much more limited) glycogen stored in the muscles and liver.

Glycogen, the stored formed of carbohydrate, is an extremely valuable commodity for an endurance athlete. We have only enough glycogen stored in the body to perform about two to four hours worth of exercise depending on intensity.  The higher the intensity, the faster glycogen is utilized.  The stored amount of glycogen in a body is roughly around 2000kcals. Once this is used up we rely solely on fat and protein as an energy source, severely compromising both endurance and intensity.  When it runs out it is called "hitting the wall" and it very much feels like it. Contrarily, we have enough fat in our bodies to complete virtually any event with energy to spare. Better utilization of this stored fat (energy) for long endurance events could give an athlete a significant advantage.

But does it work?

It appears that it does.  Consuming a high fat diet followed by a high carbohydrate does increase fat metabolism. In one study, a control group of cyclist following this protocol did increase fat metabolism- and lowered carbohydrate oxidation. Although this process raises the level of an enzyme that is important in fat metabolism, it also LOWERS levels of one responsible for carbohydrate breakdown.  Basically your body adjusts enzyme production based on the food that is being put in the body; which makes sense.

The conclusion; don't change your diet just yet as the jury is just out, but there could be a methodology down the road. The access and utilization of carbohydrate is so very important for athletic performance, risking a decrease in the ability to utilize carbohydrate does not justify the means. There may be some benefit, especially for ultra-endurance athletes, but it has yet to be solidified.

References

Peak Performance Cycling; number 227: 1-4