Aura Break inverts the effects of Fairy Aura and Dark Aura: instead of multiplying the power of Fairy- or Dark-type moves by those Abilities' normal multiplier of approximately 4/3 (or more precisely, 5448/4096), the power of moves of that type is divided by 4/3 instead, resulting in a 25% decrease in power as long as both Abilities are active.
Even if a Pokémon with Mold Breaker, Teravolt, or Turboblaze uses a Fairy- or Dark-type move, the effect of Aura Break still applies.
Outside of battle
Zygarde with the Ability Aura Break can be separated back into Cores and Cells.
In Generation VIII only, the Zygarde Cube can be used to switch Zygarde's Ability between Aura Break and Power Construct; in all games, Zygarde cannot switch between Abilities using an Ability Capsule or an Ability Patch.