The trainer in the arena is working the Hindquarters of the mustang. She puts pressure on the Hindquarters of the mustang and when the horse moves his hindquarters away, she relieves the pressure by turning away from the horse and moving to apply pressure on the hindquarters from the other side. It is important that when she wants to relieve the pressure, that she turn AWAY from the horse instead of into the horse. Thus, if she is on the right side of the horse (as she is facing the horse) then to relieve pressure, she will turn to her own left.
In this video below, the rider is getting used to the idea that she needs to put pressure on the horse's hindquarters with the whip or flag in the appropriate hand. She is also learning that any indication on the part of the horse deserves a release of the pressure. Thus, if the horse even so much as turns his head in her direction, she should give a release.
I hesitate to include this video below but I think it is best to do so because nothing is completely predictable when working with animals or students. The main thing to notice is that the rider keeps the pressure on the horse until the horse turns and faces her.
The next video below shows a very good example of the rider putting pressure on the hindquarters and then turning away from the hindquarters to relieve the pressure as soon as the horse responds. The rider will then turn completely away from the horse in a small circle as if walking away from the horse only to appear on the horse's opposite side. In this case, however, the horse kept facing her and so she returned to the original side. The horse continuing to track her and face her is a desirable thing and we reward the horse by turning away.
This is a great video of the rider putting pressure on the hindquarters of the horse and then immediately turning away as the horse moves away from the pressure. When she turns away, she walks a tiny little circle to the other side of the horse and starts over. Pretty soon, the horse is right on the riders hip, and tends to want to follow since he is pretty much tired of her chasing his butt.
In this last video, you can see that the rider is able to get pretty close to the Mustang. He simply does not want her near his hindquarters and will turn to keep her from getting there. Thus, if he never lets you near his hindquarters, then by default, he will be following you.