
Dogs (or anything with a brain) can only take in information when they are under the threshold - so not in fight or flight mode.
If you try to train (modify behaviour) while at the threshold you will have no impact.
This is why little and often wins the reactivity game. You have to very slowly desensitise and counter condition. It absolutely cannot be rushed.
Full explanation with picture guide in this course.
This is a guide on understanding what the threshold is, signs to look for, to help combat reactivity