Renamed to WebSub. A simple, open, server-to-server Web-Hook-based PubSub (publish/subscribe) protocol as an extension to Atom Standards (and RSS).
IndieWeb use: https://indieweb.org/PubSub
in use at Google - does this improve their position for Real Time Search?
their comparison of protocols positions them (and possibly RssCloud) as the only system that can be supported on a $5/mo Hosted Server system.
Apr'2012 guide by Uche Ogbuji