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The dog-breaking, especially by the man with the red sweater, brings out Bucks primal insincts of fear and rage. His senses are sharpened and feels as a sense of wildness.
I think what is brought out was that he was no longer in authority of Humans, like at Judge Miller's Place(Beginning of the book.) He now realizes that he is a Down under dog, where he has to follow the simple rule that Humans rule over dogs.