Nuno Oliveira from “An Old Master trainer to Young Trainers”
Practice shows us that a horse cannot give his back before being curved (laterally) around the inside leg.
The rider who turns the head to the inside without the horse first relaxing in the inside curve of his body only achieves a twisted and resisting position of the head.
It is the inside leg which is active, and what is necessary is that the horse turns the head around the inside leg without having more weight on either the inside shoulder or the outside shoulder.
Nuno Oliveira from “Horses and Their Riders”
One must enter the circle with the outside shoulder of the rider going forwards, parallel to the outside shoulder of the horse and not with an action of the inside rein.
He [the horse] should make circles, in spirals, making them bigger or smaller by the rider’s legs, which guide and control the horse’s body on each circle.
Diane Followell Classical Dressage Trainer