François de la Guérinière
This exercise [shoulder in] has so many benefits that I regard it as the alpha and omega of all exercises for the horse which are intended to develop complete suppleness and perfect agility in all it’s parts.
In the shoulder-in hold your body against your outside elbow.
The most common fault made by riders in doing shoulder-in is to have the horse more bent in front of the wither (or saddle) than behind it. The horse must be the same in front of the withers (or saddle) as he is behind.
Correctly done the shoulder-in gives great results as it relaxes and straightens the horse.
The most important aid [in half pass] is given by the rider’s inside leg which pushes the horse forward and is responsible for correct bend.
The position of the [horse’s] head [in half pass] should be such that the neck is not more bent than the whole body.
Don’t exaggerate the bend in the half pass. Otherwise you block the inside shoulder.
Diane Followell Classical Dressage Trainer