CouchDB: The Definitive Guide is fantastic for understanding the inner workings of most of CouchDB's features and components. It goes into adequate detail on the use of views and does a great job of explaining how they're implemented within CouchDB. That being said, the guide does not contain any solid documentation on how _changes filtering actually works.
The _changes section seems that it should contain that information but it is mysteriously absent.
How are CouchDB filters implemented 'under the hood'?