WordPress at a glance
action is not described

wp_update_comment_count action-hook . WP 2.3.0

Fires immediately after a post's comment count is updated in the database.

Usage

add_action( 'wp_update_comment_count', 'action_function_name_7424', 10, 3 );
function action_function_name_7424( $post_id, $new, $old ){
	// action...
}
$post_id(int)
Post ID.
$new(int)
The new comment count.
$old(int)
The old comment count.

Where the hook is called

wp_update_comment_count_now()
wp-includes/comment.php 2393
do_action( 'wp_update_comment_count', $post_id, $new, $old );

Where the hook is used (in WP core)

Does not used.