The Direct Rein, as its name implies, is a simple, unilateral action of the hand that produces a corresponding effect on the horse on the same side. It is also, somewhat imprecisely, called an “opening rein” or a “leading rein,” though these terms refer specifically to the rein in its respective capacities as an outside "opening rein" or an inside "leading rein," not to the rein aid in general.
This is the rider’s primary rein aid, and is most commonly used for basic bending, turns and circles where the horse’s head is gently taken in the desired direction by a rein that leads to the inside. In fact, the vast majority of rein aids a rider will apply, no matter what level or discipline one rides, will be direct rein aids, making this the foundation rein aid in all English riding.
By opening the rein to the side, a soft pressure is applied to the inside* corner (and possibly bar) of the mouth. Sometimes a corresponding push may also be made against the outside of the lips and cheek, depending on the type of bit used, though this is not the primary effect. Here, a bit with a rounded ring will have a negligible effect on the opposite side, while a full-cheek or dee-ring bit may exert a clear push on the opposite side of the head, making the directional indication even more explicit. Though this rein aid is important for horses and riders of all levels, this gentle yet effective rein is the one used almost exclusively when training young horses or those whose mouths have been ruined.
While this rein will affect the horse’s lateral flexion, it has little or no direct effect on the horse’s longitudinal flexion, and is not involved in collection. The other key effect is that the inside direct rein will cause the horse to weight the inside fore. It affects the hindquarters only inasmuch as they are inclined to follow the path of the front end, therefore, some leg and seat adjustments may be required to keep the hind end in line with the path of the front. Here the turning seat comes into play.
The key to this rein is that, as the inside hand moves to the side into the desired direction of travel, the outside hand must correspondingly move forward in order to give. Without this give, the horse’s head is unable to turn in the direction the rein is leading it. Without give in the outside rein, the opening of the inside has little meaning for the horse and will likely ignore the aid, or worse, become resistant to it.
This is a universal rein, used by beginners and advanced riders, and for horses at all stages of development and training. It is well understood by the horse; its effects are straightforward, and can safely be employed both at slower gaits as well as at speed - it can even be used to indicate direction to a horse in mid-air over a fence, when other rein aids might unbalance the horse. It is also an ideal rein for riding in the open, such as when hunting, hacking, trail riding, etc. where the finer points of equitation matter less than clear, direct aids.
In addition, it is a crucial rein in the execution of movements requiring more significant weighting of the inside forequarter. The most obvious is the turn on the forehand, where the horse flexes to the inside and concentrates his weight on the inside fore while pivoting his body around it.
As an inside rein, it has the effect of "leading" the horse around the turn. A modified version of the direct or “opening” rein may be applied as an outside rein in certain movements, such as leg-yielding, when used in conjunction with one of the other four rein aids, though this will be discussed in more detail later.
This is a one-sided aid, meaning that, though two reins are always used in conjunction, only one of them should be engaged in a direct rein aid at any given time. Generally, the outside rein will give to allow the horse freedom to respond into the inside aid.
Contrary to popular opinion, the direct rein does not apply pressure backwards, but to the side. Though the explanation most often given for this rein (as in the Wright/Morris books on riding) is a backward pressure against the mouth, this will produce none of the effects described above. This direct backward aid is what is known as a “direct rein of opposition.” As its name implies, this rein opposes forward movement in the horse, setting the horse’s balance to his inside hind rather than the inside fore, and is therefore a different aid altogether (and will be treated in a later section, The Direct Rein of Opposition.)
* “inside” refers to the inside curve of the horse’s bend, and not necessarily the inside of the arena