Auto Sync works only after you turn it on in Settings and complete the normal Fitbit and Apple Health setup in the app.
SyncFlow does not choose an exact schedule. iOS decides when background work is allowed, so Auto Sync may run multiple times in a day, later than expected, or not at all for a while.
SyncFlow requests a 60-minute minimum interval from iOS. This is only the earliest requested delay, not a promise that Auto Sync will run every hour or within one hour.
If Auto Sync result notifications are enabled and iOS notifications are allowed, SyncFlow sends the result after an actual background run finishes. Turning Auto Sync on or running a manual sync does not send this notification, and the same result is suppressed if it repeats within 60 minutes.
Background runs check today and the previous two calendar days instead of doing a full-history sync.
If iOS restricts background tasks or Fitbit needs you to sign in again, Auto Sync waits until those conditions are cleared.
SyncFlow also skips overlapping runs so manual sync and Auto Sync do not write at the same time. If you force quit the app, background tasks may stop until you open the app again.
Use manual sync when you need the latest data immediately.