The Pomeg glitch involves using a Pomeg Berry on a Pokémon whose current HP is very low. The Pomeg Berry lowers the HP EVs of a Pokémon by 10 in Generation III. Since every 4 EVs is equivalent to 1 HP (at level 100), the Pokémon's HP will decrease with each Pomeg Berry used, as long as the Pokémon has 4 or more EVs in HP. In this way, it is possible to lower the Pokémon's current HP to 0 (without causing it to faint) or a negative number.
It's a glitch that allows you to bring your Pokemon's HP to negative numbers, and it's exclusive to Emerald and Gen 4 games.
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