The rein that makes the horse straight (Beudant) does not have to be a supportive rein. Use it as needed. Important: the horse needs to give to it in the jaw (Classic German school: muss durchkommen).
It depends on the horse how to proceed from there:
One may want to do a shoulder-in on an open (direct) inner rein at maximum Längsbiegung (from C1 by the setting of the neck) plus additional flexion at C8.
It kind of draws the hindlegs along while softening the cantilevered spine (T1-L5) and the neck (C1-8) laterally. Front and hindlegs cross. N.O. suggests to add small circles, which puts weight on the inner hindleg. This results in a more upright shoulder-in with weight shifting to the hindquarters.
Soft and warped horses profit from an exchange between a double shoulder-for and a 1/4 shoulder-in, with very small variations between Längsbiegung and literal straightness. In this case the rein that makes the horse straight is active whenever needed. The horse is lead between two indirect reins, the inner pointing toward the neckbase (C8), the other indicating the slant mid neck. The no more than 20 degrees lateral movement is thus done with two closed (indirect) reins, which support the horse and prepare it for more.
There is a detail to watch here. The shoulder-for, making the horse straight, produces the traveling postition. The shoulder-in, bending the horse, produces the working position. It is necessary to change between these two lateral movements regularly and do them on both hands. Else one ends up in one position on one hand and another on the other. Make sure the horse’s jaw gives to the straightening rein and keep the jaw mobilised on the right ringfinger to invite the right hindleg to come along. This small shoulder-in/for variation tends to make the horse upright and straight. It provides a gentle subtling of the shoulders which can easily be increase and keeps the horse on the inner hindleg, no matter what. This lateral position is a universally unseful basis for turns and lateral work, for the trot and strike-offs into canter.
All depends on the horse’s strength and straightness. If compromises must be made: forward goes before sideways, upright before long and low. However, let the horse tell you. When it raises up easily the time for long and low is over. Additional subtling and forward movement can be done on the ground. This approach supports harmony and performance and suggests itself for the horse that tends towards warping up until it is straight, picks up its energetic middleline and mobilizes. After that everything becomes much easier.
