WordPress at a glance
action is not described

in_admin_header action-hook . WP 3.0.0

Fires at the beginning of the content section in an admin page.

Usage

add_action( 'in_admin_header', 'action_function_name_373' );
function action_function_name_373(){
	// action...
}

Where the hook is called

In file: /wp-admin/admin-header.php
in_admin_header
wp-admin/admin-header.php 228
do_action( 'in_admin_header' );

Where the hook is used (in WP core)

wp-includes/default-filters.php 523
add_action( 'in_admin_header', 'wp_admin_bar_render', 0 );