If your horse finds it fairly easy to cross his legs and move sideways with his body, yet he's tossing his head during leg yields, it's possible that he's objecting to your contact with his mouth. Any effort to steady his head such as by tying it down or using draw reins is simply treating the symptom rather than the cause.
1. The first thing that occurs to me is that you might be "rein-yielding" rather than leg yielding. Often when riders begin to teach their horses to leg yield, they try to move them sideways by pulling them over with the reins. As a result, their horses feel restricted and unhappy.
Your reins actually do very little during a leg yield. It's not their job to help your horse go sideways. When leg-yielding to the right, for example, turn your left wrist as if unlocking a door to ask your horse flex at the poll to the left. While flexing with your left wrist, keep your right rein steady and supporting like a side rein to prevent your horse from bending his neck too much to the left.
2. Your legs ask your horse to move over. In the above example, your left leg moves slightly behind the girth to ask your horse to go sideways while your right leg stays on the girth to insure that he goes forward as well.
3. Keep your weight balanced over the center of your horse. It's easy to get "left behind" and lean to the left. This happens partly because the horse is moving to the right and partly because some riders push too hard with their left leg. If your leg says, "move over" but your weight says, "I'm going to make it difficult for you to do so", you'll probably resort to using your reins for leverage. To counteract this tendency to lean, pretend you.re going to dismount. That is, if you.re leg yielding to the right, step down into the right iron and pretend you.re going to dismount off the right side of your horse.
4. Now let's look at the quality of your contact. Here are the ingredients that contribute to an inviting and sympathetic contact.
First, maintain a straight line from the bit to your hand to your elbow. Keep your thumb the highest point of your hand. Make sure one hand is the mirror image of the other so that you offer an even contact on both sides of the bit.
Next, establish a firm connection with your horse's mouth. "Lightness" becomes a goal only after you begin to collect your horse and ask for self-carriage. At this stage of your horse's training, a light contact means that there isn't a solid connection from his hind legs to your hands.
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Next, strive for an elastic contact by using your elbows to allow for movement - either your movement or your horse's movement. In the walk and canter, your horse moves his head and neck forward and back. So an elastic contact requires that your arms follow this movement by moving forward and back as well. The motion is like rowing a boat.
In the rising trot, your horse's head and neck is steady, but you go up and down. You need to allow for this motion by opening your elbows as you rise. Think of pushing your hands down as you rise (rather than forward as in the walk and canter) and bending them again as you sit. The motion is like a hinge on a door opening and closing.
Run over this list while you're still on a straight line, and then strive to maintain all of these qualities during the leg yield. Challenge yourself gradually and systematically by starting with a small leg yield. For example, turn down a line that is only one meter away from the long side of your arena. Before you start moving sideways, run through your "contact check list". Then keep the contact exactly the same as you move towards the track. When you can do this easily, progressively increase the distance away from the track.
Ask someone to watch your hands during the leg yields. If you don't have a ground person, peek at your hands. Of course, ideally you should have your eyes up, but if you work alone, you might have to look at your hands for a while to get feedback. Once you can see what you're doing and can feel how to make a correction, you'll have more "educated hands" and can look up again.