The photo below shows a rider who is “with the motion of the horse.” She has a very secure position because her upper body is balanced over her legs.
In the photo below, the rider’s upper body is “behind the motion” of the horse. Her legs are so far forward that they cannot support her upper-body weight. Instead, she uses the reins as a means of support. This type of prolonged “hanging on the mouth” will deaden the nerves in the horse’s mouth and make the animal ever harder to control.
The rider below is “ahead of the motion” in her upper body. The horse is in a medium frame, but the rider’s upper body is inclined so far forward that she looks as though she would be hacking a horse in the woods on a long frame. Her leg has slipped back, making it difficult for her to stay balanced in her upper body. Consequently, she is leaning on her hands to support her upper-body weight. Also, notice how far back in the saddle her seat is. She is landing on the cantle of the saddle, rather than posting right behind the pommel.
A rider can be “behind the motion” or “ahead of the motion” in the upper body without the leg being out of position. However, when the leg slips forward, the upper body usually falls back; and when the leg is too far back, the upper body usually falls forward.