Leg yield is meant to strengthen the inside leg to outside rein connection. A common mistake is for the rider to take off all their aids, and just use an inside leg to move the horse over, which ends up with the horse just basically turning counterbent and going to the wall.
You must ride only one step sideways at a time. Then one forward, one sideways, etc.. which makes you stay honest with the rest of your aids. You can't ride the horse forward without a supporting outside leg and an outside rein to keep the neck straight. Your outside knee controls the shoulder.
Then, once you can do the one step over, one step forward. Now in a steeper leg yield, without moving the inside leg back (it stays at the girth), move the haunches over more than the shoulders (by allowing with your other aids - inside leg says over, outside aids say how much and with what body part) until you can do a haunches leading yield, then a shoulders leading, then totally parallel, etc.. that you become able to adjust the horses body.
And ALWAYS have an exact end point and ride to it. Never end up on the wall before or after you want to. If you go too sideways, ride more forward, if you're going past your marker, walk and get to your point no matter what!
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