'parallel properties' reference?

I'm looking for something explaining how 'parallel properties' work,
mentioned here:

http://www.w3.org/2001/tag/2011/06/07-minutes.html

Tim: Parallel properties - in the open graph protocol on Facebook you
get a page about a movie. The RDF references "" (empty URI) is of type
"Movie" etc... Sandro says - since they use their own namespace ogp:
then we know that ogp: usage means that the referent is the principal
subject of the resource referenced.

Thanks
Jonathan