WordPress at a glance
action is not described

loop_start action-hook . WP 2.0.0

Fires once the loop is started.

Usage

add_action( 'loop_start', 'action_function_name_4640' );
function action_function_name_4640( $this ){
	// action...
}
$this(WP_Query)
The WP_Query instance (passed by reference).

Where the hook is called

WP_Query::the_post()
wp-includes/class-wp-query.php 3097
do_action_ref_array( 'loop_start', array( &$this ) );

Where the hook is used (in WP core)

Does not used.