WordPress at a glance
action is not described

woocommerce_product_thumbnails action-hook . WC 1.0

Usage

add_action( 'woocommerce_product_thumbnails', 'action_function_name_6087' );
function action_function_name_6087(){
	// action...
}

Where the hook is called

In file: /templates/single-product/product-image.php
woocommerce_product_thumbnails
woocommerce/templates/single-product/product-image.php 52
do_action( 'woocommerce_product_thumbnails' );

Where in WP core the hook is used WooCommerce

woocommerce/includes/wc-template-hooks.php 121
add_action( 'woocommerce_product_thumbnails', 'woocommerce_show_product_thumbnails', 20 );