From Bulbapedia, the community-driven Pokémon encyclopedia.
Game Sync (Japanese: ゲームシンク Game Sync) is the process of synchronizing the player's save file with the Pokémon Global Link.
Game Sync in Online feature of the C-Gear in Generation V
The C-Gear can be used to perform a Game Sync.
While the Generation V Global Link was still active, players could use Game Sync to tuck in and wake up Pokémon sent to the Dream World, which would also update the player's downloadable Musical, C-Gear skin and Pokédex skin in their game, and their Badges and Medals (Black 2 and White 2 only) on their Global Link account.
When tucking in a Pokémon, the player is asked to select a Pokémon from the Pokémon Storage System to put to sleep, which will then appear in the Global Link. A Pokémon may only be put to sleep once a day (and can be awoken up to twice). Changing the DS clock or using the game in another DS will result in needing to wait 24 hours before a Pokémon can be put to sleep again.
Game Sync in the PSS
in Generation VI
Game Sync can be set to happen automatically while saving when connected to the internet on the Player Search System if the player has registered their game to a Global Link account, but the player may turn Auto Sync off so that updates are only performed when they manually select Game Sync in the PSS.
Performing a Game Sync updates the player's recent activities and Medals on the Global Link, transfers the most recently taken photo to the Global Link, and transfers any items obtained in the PokéMileage Club to the player's game. The player may also set Game Sync to send all or none of their Poké Miles from their game to the Global Link.
Players can only sync a game once an hour. Attempting a Game Sync when less than an hour has passed since the last one happened will reliably return an error stating that "Game Sync ended because the server is busy."