A lot of the work in developing Forge Code has been working out ways of modifying WordPress’s default behaviour without hacking core WordPress files. The main reason that it’s not wise to modify core WordPress files is that performing a WordPress upgrade will overwrite any modifications you have made. If you want to retain your code modifications during an upgrade, you have to keep track of every file that has been modded, which is tedious and error-prone.
Unfortunately, as of WordPress 2.7.1, RSS feeds are not covered by the theme/template system, and the templates reside in the WordPress core. Fortunately, there are a large number of “hooks” throughout WordPress codebase which allow you to override default behaviours. These hooks make up WordPress’s Plugin API. There are two types of hooks: “actions” and “filters”. An action is a hook that simply executes a function, and a filter is a hook that executes a function with a parameter (often, but not always, text) where the function is expected to return something. It’s like a pipeline - a filter takes something in, and it has to put something out. You can do things like return an empty string based on a conditional, run a text filter to strip HTML tags, or convert from numbers to plain English month names. To use either of these types of hooks in a WordPress plugin, you “register” them with WordPress using either the