Swarm

In Pokémon ChampionsGen III

Game

Pokémon Champions

At 1/3 or less of its max HP, this Pokémon's offensive stat is 1.5× with Bug attacks.

How it works in Champions

When this Pokémon has 1/3 or less of its maximum HP, rounded down, its offensive stat is multiplied by 1.5 while using a Bug-type attack.

“Powers up Bug-type moves when the Pokémon's HP is low.”
In-game text, Scarlet & Violet

How Swarm changed from game to game, newest first.

Gen V
… its maximum HP, rounded down, its ~~dropped: Bug-type attacks have their power~~ added: offensive stat is multiplied by ~~dropped: 1.5.~~ added: 1.5 while using a Bug-type attack.
Gen IV
… 1.5× as much regular damage. Overworld: ~~dropped: Increases~~ added: If the ~~dropped: frequency of cries heard on the overworld if any party~~ added: lead Pokémon has this ~~dropped: ability.~~ added: ability, the wild encounter rate is increased.
Gen III · Emerald
… inflict 1.5× as much regular damage. added: Overworld: Increases the frequency of cries heard on the overworld if any party Pokémon has this ability.