So I am slowly starting leg yielding at the canter with my boy and thought I'd share my experience
It goes one of two ways at this point
1. Really good - and I can feel him jumping up in the canter and listening to my half halt.
2. Really bad - and we look like a drunken sailor who doesn't know what the hell he is doing on dry land anymore. This movement, now coined, the Sideways Leg Flail, is easily attainable, and can be taught to you in 8 easy steps.
1) Pick up the canter. Attempt to move sideways in the canter. Remind the horse that he does this easily enough in trot, and its time to put on his Big Boy Underwear. Nudge with your boot, get a nasty buck, and then a long suffering sigh and a slight shifting of weight sideways. Let out a cheer, because hey, there was a response, yes?
2) Curse when your horse stops dead at the wet ground in front of you. Tell him that he's 1500lbs and should be able to get through slightly damp ground without backing up and snorting at it.
3) Pick up the canter again. Stay on when he leaps sideways to avoid said wet ground. Pretend that is what you were going to do, because yay, you are now moving sideways.
4) Attempt a half halt to try to control the shoulder that is racing away. Realize that the attempt is getting you no where, and tell yourself that, hey, at least you know that your half halt isn't working...self improvement starts with the acknowledgement of the problem, right?
5) Try for another half halt. Horse gets offended at your belief that his shoulder shouldn't be flying out and feels the need to tell you that you are clearly in the wrong, proceeds to stop and gets a mutinous look on his face.
6) Mop the sweat off your face. Cajole and promise a carrot. Ask for the canter again.
7) Repeat steps 1-6.
8) Decide that cantering sideways really isn't a marketable skill anyways, and go back down to a stretchy walk, where, had we listened to the horse, we should have stayed at in the first place.
You too can attain the Sideways Leg Flail.
In my opinion, a horse is the animal to have. 1300 pounds of raw muscle, power, grace, and sweat between your legs - it's something you just can't get from a pet hamster.
You are having difficulty with this movement because your canter is not yet collecting correctly, i.e. too strung out. You are probably also not asking for the "yield" at the correct moment. When you first start this movement, only ask for about three strides of it at a time.
For the left-lead canter, begin by riding a couple of 10-15 meter circles. Now attempt a few steps of the canter leg-yield, and then go back immediately to riding the circle again. With the right lead, you might be able to do the same thing. But, many times it is easier to ride a few steps of plie, and then immediately ride a few circles. Go straight again, ride a few steps of plie, go back to the circle. Once you can effectively get just a few strides at a time, you can gradually increase that number. Every time the horse starts to tense, go back to the circle.
It is important to remember that the lateral bend for canter must be maintained correctly no matter whether you are going along a straight line, or a diagonal line. Same everything except the use of your inner thigh to direct the motion. Plie, amoung other things, is actually the beginning of teaching the horse to respond to your directional use of your legs in the canter. Once you teach plie, the next step will be half-pass. Plie right focuses on directing the horse's canter from your inside thigh, while half-pass left utilizes the directional action from your outside thigh as example.
Definitely should be no leg flailing in the canter LY. Perhaps you asking for too much sideways?
Well, from your OP he is definitely blowing through your outside rein. If you are unable to keep his shoulders straight in the movement, I would abandon the LY and circle off and work on counter flexing him to get him yielding to the outside rein. Then attempt again.
Also, if he is avoiding the crux of the exercise by flailing and having too much sideways, then abandon the LY and send him forward.
Not sure, but I suspect it is your outside hind that needs more work to stay under...not the inside hind, Also suspect that you are not bringing your inside (left)thigh properly into contact with the horse's side in order to properly use your upper, inside thigh. This implies that you are not keeping the toes of your left foot elevated, but are riding in a toe downward position. Bring your left shoulder blade back a little closer to your spine as well. This will help increase the rein contact on the horse's right side to better control the horse's right shoulder.