Imagine the fediverse like a bunch of buildings on a street with each instance having its own giant delivery crew that is constantly working. Every time a post is made, the delivery crew is sent to other connected instances to inform them of that post. Defederation is like placing a bouncer in front of your building that automatically checks the ID of each delivery crew thats trying to enter the home instances building. If an instance it has defederated from tries to send over any of its delivery crew, they are immediately stopped by the bouncer and thrown into the trash.
Obviously it’s more complicated than that with tons of actions and things that go on across the fediverse, but the basic principle is there. It’s like placing a “do not enter” sign.