Many dogs who behave aggressively toward other dogs do so as a result of learning that their barking, growling fit results in the other dog going away. Because that behavior has been successful in the past, it’s been reinforced, and the behavior has continued or increased. You need a practice dog that has been trained to not react to the subject dogs aggression.
In contrast, in a Constructional Aggression Treatment procedure, the subject dog is presented with a different reinforcement scenario. The behavior that worked so well before – barking and lunging – no longer works. Instead of making the other dog go away, it actually makes her stay close or come back! A new behavior – acting calm – now makes the “bad approaching dog” go away. So, in theory, the subject dog learns to offer calm, relaxed behaviors to make the other dog go away.
Eventually the subject dog becomes calm and relaxed because he no longer needs to act aggressively to make the other dog go away. Lo and behold, once the subject dog becomes calm and relaxed about the other dog approaching, he actually gets happy about having the other dog approach; the change in his emotional response follows the change in his behavioral response.