François de la Guérinière
If a horse refuses to move sideways in one of the two directions, it is a sign that he has not been rendered supple enough on the opposite side.
…the shoulder-in means controlling the outside much rather than driving the inside.
Only when the horse is calm and confident can he give the rider his impulsive forces which the rider can then use at will.
Nuno Oliveira on Half Pass
Diane Followell Classical Dressage Trainer