"...use at your own risk. [mixin] puts the class in a list of things to get called back by systemManager at startup."
The function that gets called is:
public static function init(root:DisplayObject):void
This gets called by SystemManager or FlexModuleFactory after the Flex Application or module is initialized.
Some indirect discussion of mixin can be found in the "Building and Deploying Flex 2 Applications" docs in Chapter 14: "Creating Applications for Testing."