This is a key event in The Call of the Wild.

Thornton's command cracked out like a pistol-shot. Buck threw himself forward, tightening the traces with a jarring lunge. His whole body was gathered compactly together in the tremendous effort, the muscles writhing and knotting like live things under the silky fur. His great chest was low to the ground, his head forward and down, while his feet were flying like mad, the claws scarring the hard-packed snow in parallel grooves. The sled swayed and trembled, half-started forward. One of his feet slipped, and one man groaned aloud.

What happened after this event?

Buck pulls the sled to the hundred-yard mark.
The crowd places bets on Buck.
Thornton yells “Mush” to Buck.
Men begin holding their breath.