This allows the dog to use peripheral vision but will not draw all attention to the door. It is best at first if the stranger is a person with whom the dog is comfortable. The goal of the protocol is to get the dog to relax when given a cue to do so, despite the fact that someone is at the door. Some people prefer that the dog be permitted to bark once or twice as a warning before being quiet. This may be possible, but for some dogs, even reacting to that limited extent may send them into a cascade of behavior that is undesirable and inappropriate.
Relaxed animals can learn, and animals that enjoy the tasks learn faster.
You should review the plan with the stranger before you practice with the dog so that you two can communicate without confusion. This helps prevent anxiety in the dog. As soon as you hear or anticipate that you will hear the knock, call the dog to look at you. If this does not work and the dog continues to react, take the dog out of the room, practice some tasks from Tier 1 when the dog is calm enough to successfully do so, and start again at a softer level of knock with more distance between the dog and the door. You can also prevent the full-blown inappropriate behavior and help the dog relax while using a head collar.