How does carbo load work?

Glycogen: Extra glucose is stored in the liver as glycogen. This is broken down later when in 'starvation' mode.

Extra Carbs: athletes now may consume the high-carbohydrate diet (55% to 60% of total calories), and eat extra carbohydrates (70%) for 3 days before an event. Carbohydrate loading regimen began with a period of several days on a low-carbohydrate diet designed to deplete stored glycogen, followed by consumption of a diet high in complex carbohydrates for 3 days before an event