If used from the Bag, it changes the Ability slot of a Pokémon from one of its standard Abilities to its Hidden Ability. As a result, if the Pokémon evolves and has a different Hidden Ability than its Evolution, its Evolution will have the Hidden Ability. A Pokémon that has its Hidden Ability due to the Ability Patch can also pass down its Hidden Ability via breeding.
The Ability Patch can only be used on a Pokémon that belongs to a species with a Hidden Ability. For example, it can be used to change a Rookidee's Ability from Keen Eye or Unnerve to Big Pecks, but it cannot be used to change a Metapod's Ability (which is always Shed Skin).
Generation VIII
The Ability Patch cannot be used to change a Pokémon's Ability from its Hidden Ability to one of its standard Abilities.
Generation IX
The Ability Patch can now remove a Hidden Ability, setting the Ability to the Pokémon's first Ability slot.
Held item
Fling fails if the user is holding an Ability Patch.