The vivofit 2 will autosync to your compatible smartphone once you reach 2000 steps within 60 minutes1 or after an activity is recorded.
In addition the following requirements must be met as well.
- The device must be in its normal page loop, not on the sleep page.
- The device will not sync when using HRM or in an activity.
- The device will only sync to connections that have successfully paired in the past.
1This count resets after every successful sync, manual or auto.
Original Garmin post here.