I want to point your attention to a great article right here: ardenpackeer.com, good stuff, deffinately worth the read. I am looking this up today, as i ran into it last night doing more BGP labs. Again, just to re-iterate. The feature gives you the ability of taking a network, thats advertised by a certain source, and inject a more specific route of this network into the BGP table (and then to the routing table).