"Riding in hand, depends on the rider's hands and not less important on the acceptance of the them by the horse." --Lee Stanek
Inexperienced riders often think of the reins as a one way communication to the horse's jaw. They may not understand the motion of the bit inside the horse's mouth or how much they throw the horse off balance with unsteady hands. Inexperienced horses tend to pull on the bit and don't understand how much more difficult they make their undersaddle experience by putting up a resistance. Ideally, the rider has light hands so as to feel the horse's mouth move in his hands, and the horse relaxes his mouth around the bit and "rests his jaw in the rider's hands."