this is a pretty neat tool. you can look up topics to find commonly blogged stories, then get the graph on how that story grew through "blogspace." essentially telling you who initially wrote about the story and who is copying each other (for example, im copying slashdot).
the graphs are pretty neat and the faq mentions they have developed a ranking algorithm to discover these "originator" nodes as the best result, as opposed to maybe a well-read, oft-linked site that copied that originating node.