The Pokémon ignores the target's stat changes when attacking, and ignores the attacker's stat changes when being attacked. It cannot ignore changes to the Speed stat, however.
If a Pokémon with this Ability uses a damaging move or is targeted by a damaging move, the other Pokémon's stat stages are ignored during damage and accuracy calculation.
Specifically, when a Pokémon with this Ability uses a damaging move, the evasion stat stages of the targets are ignored during accuracy calculation, and the Defense and Special Defense stat stages of the targets are ignored during damage calculation; when a Pokémon with this Ability is targeted by a damaging move, the accuracy stat stages of the attacking Pokémon are ignored during accuracy calculation, and the Attack and Special Attack stat stages of the attacking Pokémon are ignored during damage calculation.