It lists the libraries and their sizes in a special dialog and asks you to select which ones you want exported.
Then it pops up an error.
Hopefully that will be fixed by the time it is released, but I filed a bug report to make sure.
EDIT: Yeah, feature still in development. The fact that it was wired to the GUI already was what tricked me.
In other news, there won't be a release this week, until it's done.