In battle
Generation III to IV
When a Pokémon with Synchronize is burned, paralyzed, or poisoned by an opponent, that opponent will be inflicted with the same status problem. When a Pokémon with Synchronize is badly poisoned by an opponent, that opponent will be inflicted with the regular poison condition. Synchronize will have no effect if the opponent has a type, Ability, or field condition (e.g. Safeguard) that prevents it from being burned, paralyzed, or poisoned. However, Synchronize does affect an opponent behind a substitute.
If Psycho Shift is used by a burned, paralyzed, poisoned, or badly poisoned opponent, Synchronize activates after Psycho Shift cures the opponent of its status ailment. Therefore, Synchronize will inflict the status ailment anew in such a case.
The move Toxic Spikes does not activate this Ability unless a Pokémon on the field directly causes a Pokémon with Synchronize to switch in, such as by using the moves Roar or Whirlwind.
Generation V
When a Pokémon with Synchronize is badly poisoned by an opponent, that opponent will now be inflicted with the bad poison condition, rather than the regular poison condition.
If Psycho Shift is used by a burned, paralyzed, poisoned, or badly poisoned opponent, Synchronize now activates before Psycho Shift cures the opponent of its status ailment. Therefore, Synchronize will fail to inflict the status ailment anew in such a case.
Outside of battle
From Pokémon Emerald onwards, if a Pokémon with this Ability is leading the party, there is a 50% chance that wild Pokémon encountered will have the same Nature as the Pokémon with Synchronize. This does not apply to roaming or gift Pokémon. In Emerald, it does not apply to stationary Pokémon, such as Regirock or Rayquaza.
The Ability does not apply when a Pokémon with Synchronize is strolling in the Pokéwalker or when capturing Dream World Pokémon from the Entree Forest.