Depletion then rebound
Glycogen loading uses a depletion phase, often through lower carbohydrate intake combined with training, followed by a rebound of higher carbohydrate intake. After depletion the muscles become primed to take up and store more glucose, so the carb load can supercompensate glycogen stores above their usual level.
Endurance athletes have long used variations of this strategy to maximize fuel before competition.
Why bodybuilders use it for stage look
Each gram of stored muscle glycogen binds several grams of water, so filling the muscles fuller draws fluid into the cells and can make a physique look rounder and more complete on stage. The visual effect is temporary and tied to the timing of the carb load.
Results vary widely between individuals and manipulating carbs, water, and sodium near a contest can carry risks if done carelessly. This is general educational information about a physiological mechanism, not a peak-week protocol or medical advice.