WordPress at a glance
action is not described

comment_loop_start action-hook . WP 2.2.0

Fires once the comment loop is started.

Usage

add_action( 'comment_loop_start', 'action_function_name_8998' );
function action_function_name_8998(){
	// action...
}

Where the hook is called

WP_Query::the_comment()
comment_loop_start
wp-includes/class-wp-query.php 3184
do_action( 'comment_loop_start' );

Where the hook is used (in WP core)

Does not used.