In battle
When a stat of a Pokémon with this Ability is lowered by an opponent, its Attack is increased by two stages. Defiant will not activate if the Pokémon with this Ability lowers its own stats (such as with Close Combat) or if its stats are lowered by an ally.

If multiple stats are lowered (such as by Memento or Venom Drench), Defiant activates for each stat lowered.

Defiant activates before a held White Herb. If the Pokémon no longer has any negative stat stages after Defiant is applied, the White Herb will not have the chance to activate.

Outside of battle
Defiant has no effect outside of battle.

In battle
During a sandstorm, Sand Force increases the power of Rock-, Ground-, and Steel-type moves used by the Pokémon with this Ability by 30%. A Pokémon with this Ability will take no damage from the sandstorm if it otherwise would.

Outside of battle
It is unknown if Sand Force has an effect outside of battle.