WordPress at a glance
action is not described

core_upgrade_preamble action-hook . WP 2.9.0

Fires after the core, plugin, and theme update tables.

Usage

add_action( 'core_upgrade_preamble', 'action_function_name_5515' );
function action_function_name_5515(){
	// action...
}

Where the hook is called

In file: /wp-admin/update-core.php
core_upgrade_preamble
wp-admin/update-core.php 639
do_action( 'core_upgrade_preamble' );

Where the hook is used (in WP core)

Does not used.