From Bulbapedia, the community-driven Pokémon encyclopedia.
The Sumo Conference is an annual Pokémon Sumo tournament held annually in Rikishii Town.
The prize is a King's Rock and a year's supply of Pokémon food. Pokémon entered in the competition must weigh 80kg or more. Only basic physical moves are allowed, and the goal is to eventually throw the opponent out of the sumo-ring. If the executive of the Sumo Conference (namely Shonosuke) is present and catches a law-breaking move that the referee didn't see, he will make a protest against the referee and reveal what law was broken. Then the referee will disqualify the law-breaking trainer.
When Ash and his friends visited Rikishii Town, the annual Pokémon Sumo Conference was taking place. Ash decided to enter the competition with his Snorlax, and managed to win the competition by defeating former champion Raiden.
- There were many Pokémon that entered the competition despite being under the minimum weight qualifications. However it could be that Pokémon have varying weights in the anime unlike the games.