A send activation hook is used to change default (send) behavior of sending email activation to user when account is created. Default is true mean send.

<?php
function my_component_init(){
		ossn_add_hook('user', 'send:activation', 'my_component_hook');
}
function my_component_hook($hook, $type, $return, $params){
		 return false;
}
ossn_register_callback('ossn', 'init', 'my_component_init');

Example of hook usage is https://www.opensource-socialnetwork.org/component/view/243/user-auto-validation