Brett Slatkin and Brad Fitzpatrick demonstrate what PubSubHubbub is and how it works. PubSubHubbub is an open protocol for distributed Publish/subscribe communication on the internet. The protocol extends the Atom and RSS protocols for data feeds. The main purpose is to provide near-instant notifications of change updates, which would improve on the typical situation where a client periodically polls the feed server at some arbitrary interval.
Video (3:20 min.):