The master node maintains the official version of the system configuration. You must make all changes to the system on the master Gateway, the backup Gateway does not allow you to edit properties. Similarly, Designer only connects to the master node.
When changes are made on the master, they are queued up to be sent to the backup node. When the backup connects, it retrieves these updates, or downloads a full system backup if it is too far out of date.
If the master node has modules that aren't present on the backup, they are sent across. Both types of backup transfers, data only and full, will trigger the Gateway to perform a soft reboot.