The hooks  which are registered with the loader provide meta-data for the management implementation layer for extending it at run-time. Their primary functions are to load the detail of the XML plugin definition, map it to the implementation, and marshal the implementation callbacks to the common plugin interface. How they do this depends on the implementation itself, and ranges from registering factory methods to simply adding the items directly.
In most cases the physical object need not be loaded until the callback is invoked, since the plugin definitions provide enough contextual information to build the interface or determine when they need to be invoked.
 A hook is something you can hang your stuff on.