You can enable the option “Once synced, keep synced in background” in the Advanced tab of a sync item’s settings. After the sync has been run as usual, files will be automatically transferred as soon as they are created or modified on the source (or removed, if the option “Remove files not on source” is enabled). For a two-way sync, this also applies for the destination.
A background sync will run indefinitely and cannot be restarted until stopping it from the context menu.
Please note that after restarting the app, a sync will compare the whole source and destination again before monitoring for changes in background. While the changes for this first sync have to be confirmed by default, background changes are always applied without confirmation.
Please note that changes on a volume can only be detected when they are performed from the local Mac.