It requires all fainted Pokémon in battle, which is possible with the Pomeg glitch in Emerald. In order to perform the glitch on other versions, the player may trade an excessive HP Pokémon to another version.
Shared steps
1. Enter a Trainer battle with all fainted Pokémon.
2. Use a Revive on a Pokémon and attempt to send out this Pokémon.
3. Select 'yes' on the "is about to use [x]" message.
4. Cancel out of the menu with B.
Steps (Emerald)
1. With all other Pokémon fainted, use a Pomeg Berry to faint a Pokémon or give it excessive HP. This requires that the Pokémon has significant HP EVs (hence if it doesn't, the player can get the glitch to work by forcing it to battle Pokémon such as Whismur).
2. Heal the Pokémon if it ends up with excessive HP to give it 0 HP, and end up with all fainted Pokémon.
After performing the shared steps (above), the player will send out what is known as a "YOP" (a copy of the opponent's Pokémon).
Steps (Ruby/Sapphire)
1. The player should trade a Pokémon with excessive HP acquired from the Pomeg glitch into Ruby and Sapphire.
2. With all but one fainted Pokémon, the player should use a HP Up or Rare Candy on this Pokémon to faint it, leaving all fainted Pokémon in the party.
After performing the shared steps (above), the player may send out a Bad Egg and possibly win a Trainer battle.
Steps (FireRed/LeafGreen)
1. The player should trade a Pokémon with excessive HP acquired from the Pomeg glitch into Ruby and Sapphire.
2. With all but one fainted Pokémon, the player should use a healing item on this Pokémon to faint it, leaving all fainted Pokémon in the party.
After performing the shared steps (above), the player may send out a scrolling beige box.