what I see are a few things. I agree that you're causing an awful lot of unneeded torque on your knee. What's causing this is you're turning into the twist too early. Your torso should only start to turn away from your target as your left foo begins to cross behind your right. By doing it too early you're losing alot of the load that you would normally be using to pushing though your throw. try to keep your body from turning too early and make sure your legs and body and arm all move as one. Right now they are moving as separate pieces
your pull is pretty good, the only thing I would say about that is to keep your chin up. With your head down and not looking toward your target the disc seemed to leave your hand a little early, and that is prolly why you find throwing understable discs a lil easier, due to that fact they wil ride up out of the early hyzer and turn. Just something to keep in mind.
The big issue I see is your last step. WAY TOO LONG lol. You really don't need to extend your leg that far out. What that is doing is causing you to open up your hips too early, and therefore they are getting too far ahead of your torso and arms. Granted your hips should be moving before them, but they shouldn't be out in front. Your right knee and elbow should be in a line together, from x step and into the hit, moving together. Your hips will cause the whip of your arm and knee to come around to your release. If your hips are way out in front, your knee will open, and then you will bring your arm and torso around. You're losing a great deal of power that way, and if you add in the large step and already awkward pivoting, your knees are taking a great deal of punishment.
Your follow through is great though, don't lose that. You really throw through the hit point, which is good. Shorten your steps, tighten your x step, and make sure your body is moving all together as one unit, and you'll see big improvements. I would think.
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