WordPress at a glance
action is not described

update-custom_(action) action-hook . WP 2.8.0

Fires when a custom plugin or theme update request is received.

The dynamic portion of the hook name, $action, refers to the action provided in the request for wp-admin/update.php. Can be used to provide custom update functionality for themes and plugins.

Usage

add_action( 'update-custom_(action)', 'action_function_name_5417' );
function action_function_name_5417(){
	// action...
}

Where the hook is called

In file: /wp-admin/update.php
update-custom_(action)
wp-admin/update.php 283
do_action( "update-custom_{$action}" );

Where the hook is used (in WP core)

Does not used.