The idea of carbohydrate loading is simple - before an endurance race, increase your intake of carbohydrates to increase your glycogen stores. This can be 1-4 days before your event and can be paired with tapering your training as well.
"Runners talk about eating a big bowl of pasta the night before a race because “carb-loading” is a good way to increase glycogen levels. A lot of glycogen is used in the first 10 minutes of exercise, and the rate of glycogen use falls until it is depleted altogether.”
Carb loading has some benefit for those racing for over 90-120 minutes. Most endurance events meet that criteria, and can be 3-4 hours or ever days long. An athlete can store up to 1,800 to 2,000 calories of fuel as glycogen in the muscles and liver.
This is some recent evidence that suggests in races lasting longer than 90 minutes, maximized glycogen stores may improve a runner's finish time by 2 to 3 percent. This could mean a 5-7 minute improvement for a 4-hour marathoner. But I have only seen studies on runners so far, and i’d love to see if the effect is similar for biking or rowing, which I imagine it is.
So what is worth trying? The Sports Nutrition Handbook for Professionals recommends tapering for 3 days and increasing carbohydrate intake to 10 g/kg/day (normal daily carb intake anywhere between 5-8g/kg/day). Don’t try anything new the night before a race, but certainly experiment with carbohydrate intake and see what works best for you or speak with a Sports RD to learn more.