I'll warn you, this is a geeky post. But bear with me. You can reward yourself with chocolate if you make it all the way to the end.
It was only going to be a matter of time until someone said it. I think the first trainer to mention it to me way back when was Cherry Baumann: "Dressage is inside leg to outside rein". Sure it is. What the French does that mean though, I thought back then. I envisioned my inside leg somehow pushing the whole horse over into the outside rein. That kinda made sense. Since then I've heard and read it a thousand times. At least. And I don't know how many times you have to hear it until you get it, viscerally. But it's worth chasing it, because that really IS what dressage IS. Although I would argue that we should say "inside leg to outside aids" because that outside leg does play a role. (And at some point I will have to pay royalties to Tanya Vik because, you guessed it, my morning lesson today inspired this nuance.)
When we agree that in dressage training, we create and advance the mutual balance of horse and rider, influencing and strategically placing the horse's inside hind leg becomes essential, to ask it to step further under the center of gravity. That's the job of the rider's inside leg to activate that inside hind and the energy created thereby needs to be received and contained by the outside aids, otherwise it just shoots out the outside shoulder. (Everyone who remembers learning leg yields can relate...)
And since the point of this inside hind activation is balance, it needs to be a "supporting step" as Tanya Vik explains it. Let's say the horse runs sideways in the leg yield but not enough forward, it crosses too much with that inside hind and is out of balance. The outside rein AND leg need to be there to contain the sideways in order to create an even forwards/sideways travel. If the outside leg is not in use, i.e. open and the knee and/or pubic bones pointing outward it invites the falling (or being pushed) out and the rein alone cannot make up for that, it doesn't allow for the horse to shift its balance over but creates a horse being heavy on that rein because the rider is likely pulling on it to overcompensate.
But "inside leg to outside aids" is not just something that occurs in a leg yield. It's an exciting feeling that permeates every good, balanced stride. It's an open line of communication between horse and rider, a "shoulder in feeling" at all times, without necessarily the angle or bend, but the idea of mutual balance. Then when we ask for a medium or more collected gait, the energy can continue to flow because it also creates straightness.
So "inside leg to outside rein" to me is shorthand for: "In dressage, we work on strategically placing the horse's inside hind leg, activated by the rider's inside leg, further under the center of gravity, thereby creating energy that needs to be received and contained by the rider's outside aids in order to create and advance mutual balance." But I'm very, very certain that this would have been just as un-understandable to me back then as the original.
