Auto-sync makes sure that your contacts are automatically updated with the latest info.
You can change the settings to automatically sync your contacts once a day, week, or month. Or you can turn the Auto-Sync off and manually sync your contacts.
Auto-Sync frequency can be changed through the Sync Settings. Go to: More>Sync Settings>Sync Frequency
Note: "S ync Manually" turns "Auto-Sync" off; therefore contacts will only sync if you press the "Sync" button.