Beudant in his 1929 "Dressage du Cheval de Selle" speaks of the following rein aides.
1. The half-halt is a raising of the hand. It has the purpose to shift the horse's weight from the forehand to the hindquarters. It is effected by a raising of the sternum.
2. He speaks of the half-pressure of the hand, which acts on the horse's jaw or poll or, if the jaw is flexible on both. It usually is the action of one set of the rider's fingers, but can also be applied by both hands at the same time.
3. He indicates the indirect rein, which moves toward the horse's neck. It is effected either by a forward opening movement of the rider's outside shoulder or a sideways movement of his inner wrist. In both cases it displaces the horse's shoulders.
4. He makes mention of the direct rein, which acts away from the horse's neck and displaces the hindquarters. It is effected by a closing movement of the rider's shoulder. The direct rein can as well be applied on the outside (like in counter changes) of the horse's neck or on its inside (like in the pirouette).
5. He speaks of the turning of both wrists into the opposite direction of movement (for instance in counter changes), combined with a half-pressure on the inner indirect rein (2+3). The turning of the wrists is effected by the lower arms.
6. To make the horse straight he mentions a half-tension of the rein towards the opposite haunch, which is effected by a one-sided closure of the rider's shoulder.
This diagonal aide may have been useful to straighten the horse with a single warp before the rider's seat (T9-12). It does not straighten horses with the double, triple and/or quad warps not infrequent today. These warps occur in the croup, the neck base and the poll.
In his text Beudant more than once speaks of the difficulty to make and keep the modern horse straight. He hints at the superiority of genetically straight horses.
German classical equitation speaks of two phases to make the horse straight. First croup, hind legs and poll, while riding forward. Second neck base and ribs in bending (the so called Rippenbiegung).
Beudant distinguishes a force that maintains movement and suggests to isolate it from the force that combats contractions. Is he hinting at core energy, which - arising from proper posture - is and should be available for movement only?