metadata.Mixin

From Alex Harui of Adobe:
"...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."