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The user loses HP equal to 1/4 of the damage dealt to the opponent, rounded down. If the user knocks itself out and the opponent's Pokémon survives, then the opponent's Pokémon will not attack during the same turn. The Pokémon will not lose 1 PP for the attack it was about to use. End of turn effects like poison, burn, and Leech Seed damage will be ignored on that turn.
In RBY, this move is widely distributed and there are not many other options for Fighting-type attacks. Mewtwo is a notable user, since it can OHKO Chansey with it.
Hi Jump Kick is a more powerful alternative for Hitmonlee with less recoil.