WordPress at a glance
action is not described

start_previewing_theme action-hook . WP 3.4.0

Fires once the Customizer theme preview has started.

Usage

add_action( 'start_previewing_theme', 'action_function_name_1419' );
function action_function_name_1419( $this ){
	// action...
}
$this(WP_Customize_Manager)
WP_Customize_Manager instance.

Where the hook is called

WP_Customize_Manager::start_previewing_theme()
wp-includes/class-wp-customize-manager.php 694
do_action( 'start_previewing_theme', $this );

Where the hook is used (in WP core)

Does not used.