How did François teach buck to pull a sled?

This is actually from chapter 2. Here is the text:

Francois was stern, demanding instant obedience, and by virtue of his whip receiving instant obedience; while Dave, who was an experienced wheeler, nipped Buck's hindquarters whenever he was in error. Spitz was the leader, likewise experienced, and while he could not always get at Buck, he growled sharp reproof now and again, or cunningly threw his weight in the traces to jerk Buck into the way he should go. Buck learned easily, and under the combined tuition of his two mates and Francois made remarkable progress. Ere they returned to camp he knew enough to stop at "ho," to go ahead at "mush," to swing wide on the bends, and to keep clear of the wheeler when the loaded sled shot downhill at their heels. "Three very good dogs," Francois told Perrault. "Dat Buck, him pull like hell. I teach him quick as anything."