From Bulbapedia, the community-driven Pokémon encyclopedia.
A poster for the Whirl Cup
The setting for the competition
The Whirl Cup (Japanese: うずまきカップ Whirlpool Cup) is an anime tournament between Water-type specialists held once every three years. Trainers from all over the world come to the Whirl Islands in the Johto region to compete.
Trainers register for the Whirl Cup in Inland City on Blue Point Isle, but the competition itself is held in Scarlet City on Red Rock Isle. In the anime, the Whirl Cup was featured from Octillery The Outcast to The Perfect Match!.
Since ancient times, the Whirl Islands have been home to many different Water Pokémon, and the fiery battles seen during the competition represent the Trainers' love and appreciation for all the creatures of the sea.
All the islands in the Whirlpool Archipelago prepare months in advance for the Whirl Cup Competition. Celebrations are often held on each island. The Whirl Cup is probably the biggest attraction to the Whirl Islands. A spiral sea shell, either red or blue, is the symbol for the Whirl Cup.
It is said that long ago the Sea Heroes (Japanese: 海の勇者; also, Water Pokémon Alpha Omega) were expert Water Pokémon Trainers. They lived under the sea and understood the hearts of water Pokémon. In addition to earning a powerful Mystic Water pendant, the winning Trainers of the Whirl Cup earn this same prestigious title.
The rituals associated with the competition are kept vital and alive and are taught by Maya, the Sea Priestess, who wields the Great Sea Spirit Sapphire, which supposedly enables communication between Water Pokémon and humans.
Maya begins the Whirl Cup Competition by evoking the Sea Spirit Union, which brings together the energy from all Water Pokémon.
After six days of battling, Maya presents a Mystic Water pendant to the winning Trainer and declares him or her a new Sea Hero. The Whirl Cup Competition ends when Maya says the following words: "O, Great Sea Spirit and past Heroes of the Sea, continue to watch over the Whirl Cup Competition and safeguard the harmony of all humans and Pokémon alike."
The Whirl Cup is broken into two parts: the preliminary rounds and the World Cup Competition, also known as the final rounds.
In place of a qualifier such as a collection of Gym Badges, the preliminary rounds weed out weak Trainers; only 64 Trainers can progress to the actual Competition. In this way, the qualifying rounds are not considered part of the actual Whirl Cup Competition. The number of preliminary rounds depends on the number of total entering Trainers, and take place in several different small stadiums, on a combination of land and water battlefields.
The final rounds are all held in the majestic Whirl Cup Colosseum. This part of the Whirl Cup is filled with six days of continuous battling.
The sudden-death opening matches last for two days. Trainers choose one Pokémon each and the winner is decided when one opponent's Pokémon is unable to battle.
The winners of the first round are shuffled and randomly paired for the second round.
In the second round, each Trainer is allowed two Pokémon, and the first Trainer to knock out both opposing Pokémon will continue to the next round.
With the competition down to only 16 Trainers, the battles continue in a similar fashion as the second round. The victors move on to the next round.
Semifinals and final
The semifinals of the Whirl Cup are essentially the fourth and fifth rounds. It's not known how many Pokémon are used in these rounds and the final, although Trainers most likely use three.