Big Pecks prevents other Pokémon from lowering the Defense stat stage of the Pokémon with this Ability. It does not prevent the Pokémon with this Ability from lowering its own Defense, such as by using Shell Smash. It does not prevent Spectral Thief from stealing (i.e., resetting) positive stat changes.
Effects that decrease Defense without using stat stages, such as Guard Swap, Heart Swap, or Guard Split, are unaffected by Big Pecks.
This Ability can be ignored by Pokémon with Mold Breaker, Teravolt, and Turboblaze.
Outside of battle
Big Pecks has no effect outside of battle.