The TutsPlus.com site has a tutorial posted that introduces you to CodeIgniter's hook system and details how it all works. The tutorial goes through each hook already included and shows how to create a custom one to fit your needs.
As a CodeIgniter developer, sometimes you end up in a situation that requires you to alter the core of the framework or the execution flow to fulfill your custom requirements. Of course, it's never recommended to modify the core files as it makes the upgrade process cumbersome. Luckily, the CodeIgniter framework comes with the hooks system, which allows you deal with this scenario.
In this article, we'll start with an introduction to the hooks system in the CodeIgniter framework. Then, we'll discuss the different types of hooks available. And finally, we'll grab this opportunity to explore the creation of custom hooks.
They start with an overview of the hooks system and what kind of functionality it offers to the developer. The article then goes through each of the default ones built in to the framework including system hooks, controller hooks and "overrides" hooks. It then finishes off with a look at creating a custom hook and how to add it into the system for use across the application.