WordPress at a glance
action is not described

login_form_(action) action-hook . WP 2.8.0

Fires before a specified login form action.

The dynamic portion of the hook name, $action, refers to the action that brought the visitor to the login form. Actions include 'postpass', 'logout', 'lostpassword', etc.

Usage

add_action( 'login_form_(action)', 'action_function_name_4538' );
function action_function_name_4538(){
	// action...
}

Where the hook is called

In file: /wp-login.php
login_form_(action)
wp-login.php 489
do_action( "login_form_{$action}" );

Where the hook is used (in WP core)

wp-includes/class-wp-recovery-mode.php 95
add_action( 'login_form_' . self::EXIT_ACTION, array( $this, 'handle_exit_recovery_mode' ) );