When an attack causes its HP to drop to half or less, the Pokémon gets angry. This lowers its Defense and Sp. Def stats but boosts its Attack, Sp. Atk, and Speed stats.
When the Pokémon's HP falls to 50% or lower as a result of an attack from another Pokémon, its Attack, Special Attack, and Speed rise by one stage each, while its Defense and Special Defense fall by one stage each.