Update index.md

Update language around historical backfill to include how to ensure the backfill is running.

Author: charholt

src/unify/profiles-sync/profiles-sync-setup/index.md

@@ -117,7 +117,7 @@ If you choose to re-enable a table or property to sync again, only new data gene
 
 #### Using historical backfill
 
-Profiles Sync sends profiles to your warehouse on an hourly basis, beginning after you complete setup. You can use backfill, however, to sync historical profiles to your warehouse, as well.
+Profiles Sync sends profiles to your warehouse on an hourly basis once setup is complete. Setup is considered complete after the initial automated backfill has finished, which will sync all profile data. To initiate the backfill, the Profiles Sync pipeline requires live data flowing into your workspace. If live data isn’t available, you can send test data to the workspace to start the backfill sooner. You can also use backfill to sync historical profiles to your warehouse.
 
 > info ""
 > You can only use historical backfill for tables that you enable with [Selective Sync](#using-selective-sync) during setup. Segment does not backfill tables that you disable with Selective Sync.