I would like to take some time to discuss the idea of length of rein, a subject that is oftentimes debated, usually oversimplified, and extremely pertinant to the art of equitation.

Once in a while, we see riders guiding their equine partners with the aid of little or no rein at all. This is oftentimes a great thing for, as I have previously stated, the horse should be carried by the rider's leg, lifted underneath itself if you will. That being said, there is a fine line between too short and too long of a rein, all dependent of course upon the individual rider's skill set and the horse that they are riding. Although a long rein is oftentimes a sign of a well balanced individual and a rider who is lifting their partner's ribcage with the use of their calf muscles, too slack of a snaffle or curb can also be a sign of inexperience and a lack of understanding for the way in which our reins function. The reins are not, as is predominantly assumed, a steering device. A rider who is in commune with their horse, guides him more by shifting body weight between the inside and outside stirrups than by steering the horse from the right to the left in a similar style to a trucker, maneuvering their tractor trailer.

Regardless of rein length, a rider is at her best when sitting in the middle of her horse, elbows at her side and shoulers back. I never cease to find connections between the art of equitation and the properties of alignment taught in hatha yoga.
Hundreds of thousands of years ago, human beings were physically structured in a manner closer to that of the horse. One particularly pertinent similarity between early human Hominids and the Hippidion Equus is the existence of a tail. For horses, this attribute obviously remains, and as for us humans, well, we have a tail bone. It is that very tailbone that is the root of equitation, for it is in dropping our seat and "sitting deep" if you will, that we riders stabilize a connection to our horses, via our core center of balance. Once we're balanced, the rest easily falls into place, given proper guidance of course. Our balance must come from our seat, not our hands and not, by extension, our reins. Through our seat, we also find a source of steerage that is often assumed to come from the reins. By shifting weight in between the respective seat bones, the rider sends signals to their partners regarding direction and the support needed for the shift of weight needed to corner or change direction in a well balanced fashion.

Getting back to my original point here, when a riders reins are too long, it gives them little chance to be well balanced throughout the rest of their body. How can one correctly drop their tail bone and lift their upper body when their elbows are extending out behind them? And of course, should the reins get too long, a rider's hands will naturally rise up towards their shoulders, causing the elbows to sprawl out. This makes sense as, should the reins get long and the riders hands travel towards their stomach, they will inevitably hit a barrier (the person's body). The longer the reins, and the more "jacked up" the rider's elbows become, the less control they will have.
In other cases of excessiely long reins, the riders hands travel back towards their (not to sound crude) crotch. When this happens, the rider's upper body falls forward in compensation. The result is, again, a lack of balance on the rider's part, which contributes to a lack of balance on their partner's part. The end result is excessive speed and lack of organization.

This essay has been rather long winded, and I fear that I could write even more. It can actually be quite challenging to keep one's reins properly shortened. Horses, especially the great ones, are seldom easy to ride, and have a tendency to try and take advantage of their rider's limited strength by leaning against the rein pressure in an attempt to gain a slackened rein, and therefore, more control. To remedy all this, I suggest a simple mechanical study of the cause and effect understanding of equitation as outlined above. It is amazing the difference that a simple shift of consciousness to one's own body as working in conjunction with their horse rather than as a seperate entity can make. Start practicing yoga and then looking for connections between your practice and your riding...start taking time before each ride to center yourself in the saddle, visualizing the core line that runs down your center, connecting your shoulders to your seat bone, to your heels, and most importantly, to your horse.