I think the author intended to mention installing two doors in an airlock style vestibule to help prevent hawks from flying between your legs when you enter your chickens' run or pen. Open the outer door, enter the vestibule and close then lock the outer door. Next sweep or search the vestibule carefully to ensure that no hawks have slipped inside with you. Only after ensuring that the vestibule is hawk free is it safe to open the inner door and enter the run or pen.For those who say fishing line and CD's, or a decoy owl, or whatever, works to deter hawks, here's an interesting video. I think the only way to stop them is completely covering your run.
Now that was a really bold or hungry hawk. I have had Coopers hawks hit my aviaries after pigeons while I was standing there. One also attacked a group of birds feeding at my feet. I had a broom in hand and nailed a home run.