WordPress at a glance
action is not described

before_woocommerce_pay action-hook . WC 1.0

Show the pay page.

Usage

add_action( 'before_woocommerce_pay', 'action_function_name_5802' );
function action_function_name_5802(){
	// action...
}

Where the hook is called

WC_Shortcode_Checkout::order_pay()
before_woocommerce_pay

Where in WP core the hook is used WooCommerce

woocommerce/includes/wc-template-hooks.php 313
add_action( 'before_woocommerce_pay', 'woocommerce_output_all_notices', 10 );