Life with horses is never dull. There are so many disciplines, breeds, jobs in the equine industry, ways to train, teach and ride. It's a journey where always learning more is a goal.
What do you understand about the use of the outside rein?
In thinking about how to answer, it became clear that this is an aid with so many componets to wrap your mind around. It's not just the outside rein that makes things happen, but a combination of aids working together. Horses are very sensitive creatures. They feel the slightest adjustment of the aids as well as feeling (and ignoring) loud overbearing aids. We, ever learning riders, tend to ride with loud aids. (me included) It is critical to learn how to be quiet with our aids, thus becoming more effective.
The outside rein is several things- regulates speed, tells direction, and helps balance.
Regulates speed--If your horse is trucking around like a freight train, ie rushing, your first instinct may be to pull on both reins to slow down. Direct rein contact on both reins at the same time typically means stop. Since we only want to slow down and regain balance, using the outside rein for a few strides will help accomplish this. But-to make it even more effective and only require a few strides to get the job done, your seat must be involved as well. If he's trotting around 90 miles an hour and your are posting 90 miles an hour with him, then you are going to go 90 miles an hour. Slow your post and give the outside rein aid and the horse will then start to match YOUR speed.
Tells direction-the outside rein tells the horse where to put his feet. The horse travels around the arena and cuts the corner, or just knows to follow the outside track. You want to ride 2 or 3 steps into the corner. Your outside rein aid tells him to keep going straight ahead...but to be effective correctly it requires also using your seat and leg. Your body needs to continue to ride straight ahead, outside rein saying straight ahead and the inside leg to keep the barrel straight. Too much outside rein will create a crooked horse that cannot make it into the corner. His neck will be overbent to the outside and the barrel will be too far to the inside and the only way he can keep his balance is to fall in. So that is going straight, how about turning? Most riders are taught to pull on the left rein to turn left. That is fine for the novice rider, but take it up a notch and now you want to ride a perfect 20 meter circle, no eggs, ovals, or corners! The inside rein creates the bend, but that's it and you ask for that before you start the figure. When you make the turn, it comes from your seat, turn your hips. The rein aid will happen as long as your arms are connected to your body...then the upper body becomes part of the seat. When you turn your seat, your arms and shoulders follow the turn as well, thus giving you the outside rein to make the turn. Again the leg is needed as well to keep the haunches in line. This follows into the next use--helping balance.
Take the 20 meter circle, it starts out good, then the second half the horse starts to drift off the circle or fall out. Chances are your arm has extended forward and you have no outside rein at all. There was a hole in your aid and the horse fell right thru it. (they are so good at finding the holes in our aids, aren't they :) That is why there needs to be a bend in the elbow and held next to the body...so it is flexible and can give and take the rein as needed and next to the body so it becomes part of the seat. So when you have that rein connection, then your half halts become more effective, which is the balancing aid.