What to do with that outside rein?

There has been a lot of confusion among riders about what to do (or not to do) with the outside rein. Many riders that I meet have learned to ride the horse 'on the outside rein'. Often this is interpreted as keeping the outside rein short and keeping the horse (too) straight. This interferes with the bending and suppleness of the horse. On the other hand, riders that work too much with their inside hand and do too little on the outside create too much neck bending in their horses which causes them to fall over the outside shoulder. The outside rein in Academic perspective:

When the goal of your training is to supple your horse up through straightness training exercises and then to collect the horse, you first need a horse that can bend to both sides evenly. When it can bend evenly, you can straighten it. In this context straight means: with hips and shoulders aligned. Straightness training exercises are necessary to conduct the power generated by the hind legs through the body in a biomechanically correct way. When the shoulders are in front of the hips, no matter if the horse is on a straight or on a curved line, the energy can flow from back to front. It creates a balanced horse that can move supple and elegant. The suppleness is achieved by bending the horse to both sides, stretching the muscles along the spine. With a stiff horse, that is why in first instance we focus only on the inside rein (bring your nose in) and the inside leg (bend around me). This we alternate to left and right. Consider a stiff garden hose with some kinks in it. By bending it gently to left and right we supple it up and smoothen the kinks. Then we can shape it in a curve and allow the water to flow through it fluently. Any kink would block the flow. 
Now, when the horse can bend around your inside leg, sometimes you will notice that it is getting too much and that the horse breaks out over the outside shoulder. Now there is the job for the outside rein: to reduce bending when it gets too much, and to guide the flow of energy around the bend towards the mouth of the horse and into your hands. When the horse bends too much in its neck, it is as if the garden hose got a kink in it and the power of the hind legs gets partially lost over the outside shoulder. It is the outside rein that smoothens it out. You bring the outside of your body through the curve by allowing our outside ribs and shoulder to move forward, and by touching the outside shoulder of your horse with the outside rein (we call this indirect rein), you place the shoulders inwards, in front of the horses hips. If you start using your outside rein before you have bending, you limit the amount of suppleness and flow that your horse can develop. The outside rein is a reducing and a guiding rein, do not reduce or guide something that is not there! A too short outside rein or backwards working outside hand keep the horse stiff and prevents the back muscles to supple up. That is why in the Art of Riding we work in phases. In the first phase with a stiff horse, we work on inside leg and inside rein to achieve bending and suppleness. Only in the next step we reduce and guide with the outside rein. When we can bend the horse only from the seat, then we leave the inside rein and only guide the horse from the outside rein. A pitfall is that sometimes people get stuck too much in the first phase, pulling the inside rein and bending the necks of their horses too much. So in the end, it is the outside rein that is most important! However, not with every horse we can start with it. That is why often we need to start with phase 1, the inside aids. But always keep the end goal in mind: a horse that is supple and balanced and can bend from your seat. Start using the outside rein as soon as you can to guide your horse through turns. Distinguish direct rein (placing the head) and indirect rein (placing the shoulders). Only use the outside rein as direct rein when the horse bends its neck too much and otherwise use it as an indirect rein placing the shoulders in front of its hips (guiding the horse). This allows the energy to flow freely from the hind quarter into your hands.

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