Hi,
After the payment on WorldPay's payment page, the customer is redirected back to the website to the payment notification URL of the WorldPay plugin:
index.php?option=com_hikashop&ctrl=checkout&task=notify¬if_payment=bf_rbsbusinessgateway
This calls the WorldPay payment plugin which checks that the notification is valid, confirms the order and then redirect the customer to the return or cancel URL based on whether the payment was successful or not.
Also, make sure that you enter <WPDISPLAY ITEM=MC_callback> in the call back URL setting of your WorldPay merchant account.