Topology mirror in 2.5 - How to use?

In edit mode from 2.5, below “x-mirror” there is a checkbox for “Topology Mirror”. However, it does not seem to work at all. And i don’t understand what the pulldown menu below is for, “Edge Tag”.

Does anybody know how to get this working?

i would also like to know what topology mirror does?

And i don’t understand what the pulldown menu below is for, “Edge Tag”.

You can mark edges as a seam/crease/bevel etc by Ctrl by clicking in the edges by selecting which option you want. This makes it very fast to mark edges.
Demo http://www.screencast.com/users/blenderwho/folders/Jing/media/8639bf87-6f8a-489a-b26b-83c6dea64dd1

Supposedly, when having both x-mirror and Topology Mirror checked, it should be possible to edit the mesh x-mirrored even when it’s not symmetrical, as long as the topology is symmetrical.

But doing tests isn’t giving me satisfying results atm. Perhaps I’m completely wrong…

That’s what I also believed to be the case but never been able to get it to work. X-mirror seems ok but topological not.