QNAP offers to connect an external USB or eSATA drive and start a pre-defined backup job immediately after automount. In addition, the device is umounted after the job has finished. The "eSata" LED shows if the devices is still mounted (and the job is still running).
Up to that point, the concept suits my requirements perfectly.
What I am missing is a versioning. And it this should be efficiently consuming the drive space. Technically speaking, I would like to see the usage of hardlinks for files that did not change and a versioning by directories named by date.

I could just install some rsnapshot script plus udev rule for automounting. But I would like to understand how the internal mechanism works.

When the device is plugged, it is mounted under
/share/external/...

If a backup process is configured via the QNAP backup manager,
ps xa shows
qsync -j:Job65 -c:/mnt/HDA_ROOT/.config/qsync/extdrv.conf

Furthermore,
/usr/bin/qsyncman
is running.

This service is started on boot-up via
/etc/init.d/qsyncman.sh start

Configuration files can be found in
/etc/config/qsync

/etc/config/qsync/extdrv.conf contains a list of all configured backup jobs (mapping of directories to an external medium).
/etc/config/qsync/qexthost.conf lists all the known external media (USB drives) incl. drive ID ("INetAddr"), file system ("FtpPassword"), volume name ("FtpAccount"), hardware vendor ("Password"). The field names ae misused here.
/etc/config/qsync/qsyncd.conf defines a port 8899.