The dog must learn that his barking controls the helper. Whether the dog is worked in prey or defense or aggression, that fundamental is the same. The prey dog is telling the helper to give him the toy, the defense dog is telling the helper to back off and the aggression dog is dominating the helper through his bark.
How that whole process is initiated depends on the temperament and drives of the dog, skill of the helper and goals of the training. Some teach this concept initially in prey, where barking flushes the helper making the helper (or rag or tug or whatever) move in response to the bark. Some teach it initially through suspicion, where the helper presents a threat or challenge, which is then neutralized by the dog. When the dog barks strongly, the "bad guy" backs down or leaves. But no matter how it is approached, the end goal is the same.. for the dog to learn he can control the helper, and in doing so start the action, counter attacks, subdue the threat, and basically run the whole show not just through biting, but also through barking.
The mailman analogy is a very good one. From the dog's perspective, since he has no way of knowing anything else, the sequence goes like this: mailman came, dog barked, mailman left. To the dog his actions caused the mailman to leave. He controlled, overpowered and scared off the mailman, driving him away from the territory, through his barking.