Every time the HP of a Pokémon with Berserk drops below half, its Special Attack is raised by one stage.
If the HP of a Pokémon with Berserk drops below half but is about to be restored to above half due to a held item such as a Sitrus Berry, Berserk will activate before the HP is restored. However, if the move that hits the Pokémon with Berserk is a multi-strike move, the item will be consumed immediately after the hit that drops the HP to below half, so Berserk will only activate if the HP is still below half after all the strikes and after the activation of the held item.
If the HP falls below half due to indirect damage, such as one caused by Leech Seed, status conditions or weather, Berserk won't activate. Berserk will not activate if the Pokémon's HP falls below half by a move affected by Sheer Force.