When a Pokémon with this Ability faints due to damage from a move that makes contact, the attacking Pokémon takes damage equal to ¼ of its own maximum HP. This Ability will not activate if a Pokémon with Damp is on the field.
In Pokémon Super Mystery Dungeon, Aftermath activates when hit by a contact move from an adjacent enemy, dealing damage equal to 25% of the attacking Pokémon's maximum HP. Aftermath will also cause the Pokémon to explode when it is hit by an explosion, dealing damage equal to 25% of the Pokémon's maximum HP to itself and surrounding Pokémon, as well as destroying all walls, items, and special tiles.
All Pokémon that are able to have this Ability can learn Explosion. Aftermath's similarity to Explosion is more apparent in languages other than English, including the original Japanese, in which the Ability is known as ゆうばく Yūbaku (Induced Explosion). This is also why the Ability Damp affects Aftermath.