How does the writer show Thornton's power over Buck?

Thornton's power over Buck is can only be summed up as the power of love. The relationship between Buck and Thornton is a relationship based upon love and trust. Thornton is the perfect master, and Buck is the adoring dog.

This man had saved his life, which was something; but, further, he was the ideal master. Other men saw to the welfare of their dogs from a sense of duty and business expediency; he saw to the welfare of his as if they were his own children, because he could not help it. And he saw further. He never forgot a kindly greeting or a cheering word, and to sit down for a long talk with them--"gas" he called it--was as much his delight as theirs.

Buck's love was expressed in adoration. While he went wild with happiness when Thornton touched him or spoke to him, he did not seek these tokens.

For a long time after his rescue, Buck did not like Thornton to get out of his sight. From the moment he left the tent to when he entered it again, Buck would follow at his heels. His transient masters since he had come into the Northland had bred in him a fear that no master could be permanent.

For Thornton, however, his love seemed to grow and grow. He, alone among men, could put a pack upon Buck's back in the summer traveling. Nothing was too great for Buck to do, when Thornton commanded. One day (they had grub-staked themselves from the proceeds of the raft and left Dawson for the head waters of the Tanana) the men and dogs were sitting on the crest of a cliff which fell away, straight down, to naked bedrock three hundred feet below. John Thornton was sitting near the edge, Buck at his shoulder. A thoughtless whim seized Thornton, and he drew the attention of Hans and Pete to the experiment he had in mind. "Jump, Buck!" he commanded, sweeping his arm out and over the chasm. The next instant he was grappling with Buck on the extreme edge, while Hans and Pete were dragging them back into safety.