It is very simple. You simply imagine the runway of the airfield you are approaching extending across the Heading Indicator. So long as your Heading Indicator is properly adjusted, when you look outside you will see that the runway is in the same orientation as you visualized it on the Heading Indicator. Using this technique can keep you out of trouble by helping you line up for the right runway, as you can work out where it is long before you can see the runway numbers on the tarmac.