Anchor Guy,
The IBOP is located on your top drive above your saver sub. It gives you a way to close off your drill string. In some cases you would be using variable rams or pipe rams in your ram-type preventers. So your drill pipe would still be downhole and you could close your pipe rams whihc would isolate the rest of the well bore, but you would still be seeing formation pressure up through your drill string, so they came up with the IBOP.