This callback triggered before the actual action code is executed. This is useful to do checks before actual action executed. One of the example is captcha :)

<?php
function my_component_init(){
		ossn_register_callback('action', 'load', 'my_component_cb');
}
function my_component_cb($callback, $type, $params){
		//$params['action'] contains the action name
		if($params['action'] == 'user/login'){
			//do some stuff......	
		}
}
ossn_register_callback('ossn', 'init', 'my_component_init');

https://www.opensource-socialnetwork.org/component/view/1549/captcha