You can enable the option “Keep synced in background” in the Advanced tab of a sync item’s settings. After the main sync has been run as usual, files will be automatically synchronised as soon as they are changed on a source base.
A background sync will run indefinitely. If you want to run the main sync again, it has to be stopped first from the context menu.
Please note that after restarting the app, the main sync will have to be run again, comparing the whole source and destination bases, before monitoring for changes in background. While the changes for the main 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.