Why, when someone has won without question, do we say that he did it “hands down”?

To win hands down has nothing to do with placing a winning hand of cards face down. Instead, the expression comes from the earliest days of horseracing. If a horse had proven its superiority and was approaching the finish line well ahead of the pack, the jockey would release the reins, giving the animal free reign to the finish. He therefore would win the race “hands down.”