In horse racing, to get the horses to run faster, the jockeys usually pull hard on the reins that are tied around the horse’s head. In doing so, the jockey has to position his hands forward. If the jockey lets go or loosens the tightness of the reins, the horse will likely slow down, switch to a trot, or come to a stop. In the 19th century, it was observed on occasion that when one horse was so much faster than its competitors, and had left such a large distance between itself and the rest of the pack, the horse would be guaranteed to win the race even if the jockey loosened his grip on the reins. 

Sometimes, if a jockey was totally sure that his horse was going to win no matter what, he would completely let go of the reins, and put his hands down as a sign of pride or self-confidence. In such cases, it would be reported that the horse and jockey had “won hands down.” That’s the origin of the expression “to win hands down,” which means “to win very easily, or with little effort.”