It takes advantage of right-of-way to avoid collision while backing.

I’m following you. When you decide to back in to your parking space, you have the right-of-way over the lane until you have completely left it. I have to yield to you, even if you come to a complete stop in the lane of traffic. While your vision and attention is compromised due to backing, I am responsible for avoiding you.

When you are attempting to back out of the parking spot and into my lane, you do not have right-of-way until you are fully established in the lane. Despite your vision and attention being compromised due to backing, you are also responsible for avoiding me. I don’t have to yield to you until you are completely within the lane.