Matches are best-of-three. After each game you may swap spells one-for-one between your main book and your 16-spell sideboard, trading within the same Circle. Lost to a wall of counters? Bring in spells that punish them. Facing an aggressive rush? Side in your defensive answers.
Sideboarding turns a single duel into a three-game chess match of reads and adaptations, one of the most skill-intensive parts of the game.