WordPress at a glance
action is not described

send_headers action-hook . WP 2.1.0

Fires once the requested HTTP headers for caching, content type, etc. have been sent.

Usage

add_action( 'send_headers', 'action_function_name_5989' );
function action_function_name_5989( $this ){
	// action...
}
$this(WP)
Current WordPress environment instance (passed by reference).

Where the hook is called

WP::send_headers()
wp-includes/class-wp.php 506
do_action_ref_array( 'send_headers', array( &$this ) );

Where the hook is used (in WP core)

Does not used.