I can't find any documentation on it and I can't get it to do anything. Basically I need to create my own mirror for updates.

I guess it is still not implemented for about ten years now
http://forum.mikrotik.com/viewtopic.php?t=7263

Why not? It is a old feature where you can add a router as a package source;
http://wiki.mikrotik.com/wiki/Manual:Up ... to-upgrade

Is there any documentation on this mirror mode? Couldn´t find anything in the wiki or somewhere else so I thought it is still not implemented.

Why not? It is a old feature where you can add a router as a package source;
http://wiki.mikrotik.com/wiki/Manual:Up ... to-upgrade

Not sure I follow. The link doesn't mention anything about "/system upgrade" just "/system package upgrade".

Are you saying that if you add a mirror under "/system upgrade" "/system package update" will use it?

This is all on 6.30-6.31 BTW.

Are you seeing the same things as I? The command line corresponds the Winbox layout

Ok, I think I have it partially figured out. "/system upgrade" apparently needs the winbox service on the host router which I routinely disable. After enabling it, I can download packages. So what does "mirror" do?