Simple

Introduced in Generation IV

Each stage of this Pokémon's stat modifiers acts as two stages. These doubled stages are still limited to a minimum of -6 and a maximum of 6. This Pokémon can still accumulate less than -3 or more than 3 stages of stat modifiers, even though the extra ones have no effect after doubling.

“Doubles the effects of the Pokémon's stat changes.”