How does London create suspense in the scene in which Buck pulls the thousand-pound load?

Buck's amazing feat of pulling a thousand-pound load by himself is a suspenseful scene. It begins with Thornton socializing in a saloon and falling into a bragging match about sled dogs. Thornton brags, without any proof, that Buck can pull a thousand-pound load all by himself. As Matthewson proposes the wager and works out the details, suspense builds. London lets readers hear Thornton's internal dialogue. He has been caught in a boast and is reluctant to back down, but he realizes what an astounding feat it would be. Suspense builds when Thornton has to borrow the funds to put up against his challenger. If he loses, he'll be two thousand dollars in the hole.
When the men move outside for the test, it's -60 degrees, and the sled runners are frozen to the snow. Men who had laid odds at two-to-one against Buck now lay them at three-to-one. And there are no takers. But when Buck takes the harness, the men are awestruck at what a fine specimen he is, and odds go back down to two. Matthewson offers to increase the wager, and Thornton and two friends add their remaining two hundred dollars to the pot. All this tinkering with the wager increases the suspense. So much is riding on Buck's performance.
London's descriptions of Buck also add to the suspense. He describes him as if every bristling hair is stocked with power. When Thornton leans in to Buck and says, "As you love me, Buck. As you love me," readers are pulled in. Will a dog perform an impossible feat to express its love for its master? Readers⁠—especially dog lovers⁠—want to believe that's so.
The description of Buck's performance in the contest also builds suspense. London describes it incrementally, so readers, like the men in the story, concentrate on every minute movement. First, Buck breaks the sled out of the ice on one side, then on the other. The clear visual imagery of how Buck moves the load is suspenseful. Readers see Buck's muscles and posture, as well as Buck's technique: "His feet were flying like mad, the claws scarring the hard-packed snow in parallel grooves." London describes the sled moving jerkily at first and then smoothly to the finish line.
Like the men, readers will probably let out a breath at this point, not realizing they've been holding it due to the suspense of the passage.