[development] Take Over Page Output from Module

The Print module, Book module's export functionality, the s5 module, they all have this trick of taking over the entire page output for their own theme function. I am building a module that is headed toward a more generic mechanism for funneling content through a pluggable template, but despite pouring over the code I can't see the magical place where all other Drupal markup is dropped in favor of the HTML document described in my own template file. What's the trick? I have a menu callback along the lines of "present/%node" and it leads to an export function that returns my theme function, which is a template outputting a complete HTML document.