WordPress at a glance
action is not described

before_signup_header action-hook . WP 4.4.0

Fires before the Site Signup page is loaded.

Usage

add_action( 'before_signup_header', 'action_function_name_5367' );
function action_function_name_5367(){
	// action...
}

Where the hook is called

In file: /wp-signup.php
before_signup_header
wp-signup.php 50
do_action( 'before_signup_header' );

Where the hook is used (in WP core)

wp-includes/default-filters.php 520
add_action( 'before_signup_header', '_wp_admin_bar_init' );