In battle
Synchronize passes on status problems to the opponent that caused the problem. Synchronize will not pass on the Sleep or Frozen status conditions, or if the status condition was obtained from items such as the Flame Orb. Synchronize will have no effect if the other Pokémon has a type or ability that prevents them from being Poisoned, Paralyzed, or Burned. The badly poisoned condition is passed on as regular poison.
Outside of battle
From Emerald onwards, if the Pokémon with this ability is leading the party, Synchronize gives a 50% chance of encountering a wild Pokémon with the same nature. Strangely, although Synchronize does not have this effect on stationary legendaries in Emerald, it has worked on all Pokémon in every game released since.