WordPress at a glance
action is not described

preprocess_signup_form action-hook . WP 3.0.0

Fires when the site sign-up form is sent.

Usage

add_action( 'preprocess_signup_form', 'action_function_name_3053' );
function action_function_name_3053(){
	// action...
}

Where the hook is called

In file: /wp-signup.php
preprocess_signup_form
wp-signup.php 886
do_action( 'preprocess_signup_form' );

Where the hook is used (in WP core)

Does not used.