The rider has no power one-handed

The rider has no power one-handed

But it takes a long time to make a colt understand that he is to turn to the right when the left rein is pulled; and if any horse resists, the rider has no power one-handed, as the reins are usually held, to compel him.

The practice of one-handed riding originated in military schools; for a soldier has to carry a sword or lance, and depends chiefly on his well-trained horse and the pressure of his legs. No one ever attempts to turn a horse in harness with one hand, although there the driver has the assistance of the terrets, and it is equally absurd to attempt it with a colt or horse with a delicate mouth. Of course, with an old-trained hack even the reins are a mere form; any hint is enough.

Fonte: Tamming Horses