‘MORE INSIDE leg!’ or ‘more outside rein’ is a common comment by dressage trainers. What does it really mean?
It means that your inside leg creates the energy when riding, and your outside rein regulates it.
The inside leg, being at the girth, asks the horse to bend around it. Then the outside rein is there to elastically maintain the horse’s shape around the inside leg, by supporting the inside aid so the horse doesn’t move away from the leg.
Your inside leg and outside rein should constantly be talking to each other and coordinate.
If you were to only use the inside leg for instance, without the support from the outside rein (and leg), the horse would move away from the leg instead of around it, and he would fall out through his outside shoulder.
Of course, it’s not just the inside leg and outside rein that’s doing all the magic. They are just individual instruments in the orchestra of aids that combined creates the symphony of harmonious riding. But they do play important parts of the beat.
If you want to be sure you’re riding on your outside rein: Do a shoulder in, and let go of the inside rein. In walk, trot, and canter.
The movement should not be disturbed if you’re coordinating the aids correctly.