WordPress at a glance
filter is not described

login_body_class filter-hook . WP 3.5.0

Filters the login page body classes.

Usage

add_filter( 'login_body_class', 'filter_function_name_7433', 10, 2 );
function filter_function_name_7433( $classes, $action ){
	// filter...

	return $classes;
}
$classes(array)
An array of body classes.
$action(string)
The action that brought the visitor to the login page.

Where the hook is called

login_header()
wp-login.php 171
$classes = apply_filters( 'login_body_class', $classes, $action );

Where the hook is used (in WP core)

Does not used.