Pokémon Red Version, Blue Version, and Yellow Version - Fainting and Switching Out
If a Pokémon faints at any time during its attack segment, including the end of its attack segment, any end-of-attack effects are applied to the Pokémon before it faints if the opposing Pokémon hasn’t fainted and the fainting by the Pokémon occurred before the end of its attack segment and didn’t occur as part of the use of Selfdestruct or Explosion (the end-of-attack effects are poison, burn, and Leech Seed); no end-of-attack effects are applied to the new Pokémon; and if the Pokémon struck first, the opposing Pokémon’s attack segment is skipped. (If the opposing Pokémon used Hyper Beam on the previous turn, it "must recharge" on the next possible attack segment.)
If a Pokémon faints before the end of an opposing Pokémon’s attack segment, usually as the result of an attack, no end-of-attack effects are applied to the opposing Pokémon.
When a Pokémon switches out, the Pokémon that replaces it does nothing during its attack segment but any end-of-attack effects are still applied to that Pokémon.