When must a player present their decklist before a tournament starts?

A player must present their decklist prior to the start of the event to ensure that all participants are aware of and can verify each other's deck compositions, maintaining the integrity of the tournament. Presenting the decklist at this time allows for necessary checks to be made against the format's legality, ensuring that all decks comply with the rules and restrictions. This step also facilitates the pairing process, as players will be matched according to the decks they've registered.

Submitting the decklist after the tournament has begun or during matches would hinder the ability to address any issues that might arise regarding deck legality. Presenting it during practice sessions would not fulfill the requirement of transparency necessary for the tournament environment. Thus, the rules establish that the appropriate time for presenting the decklist is before the event officially begins.