@bluejeff
Basically safe and solid.
But a few things I notice that could be refined:
I agree about overextending the low back at the top. And I agree about sitting back into a deeper hinge, which would require more knee bend (but with the focus on getting hips back further, with the knee bend as a consequence of that).
Your knees are also very snappy. You don't want to be slamming your knees back into a locked out position. Fully extend smoothly and then contract the quads ("pull up the knee caps") and glutes as you hit full extension.
You also look like you
might be trying to push your arms forward with your hips. You want your arms and torso to move together. You don't want any sort of collision or pressure between them. The hip drive transfers power through the shoulders and down the arms to the bell, not by the hips pushing on the arms.
Enjoy the float. You seem so concerned with keeping your shoulders packed down that you don't let your arms come up with the bell. There's a rhythm of tightness and relaxation, not just relaxed or tight overall, but relaxed where and when you need to be and tight where and when you need to be. Once you finish your hip drive, let the bell float. You can let your arms come up freely without shrugging your shoulders up or letting them roll forward. This might also be contributing to the lumbar extension because if you're aiming for the bell to get to chest height, but you're restricting it from getting there by keeping your arms locked down, the natural tendency is to get the bell height back by leaning back further.
You have some bell flipping at the end of the backswing, which indicates that you're driving your hips forward while the bell is still moving backward (and is also a consequence of a shallow hinge). This wastes a lot of power because you're using forward hip drive to brake and reverse the backward movement of the bell. A deeper hinge and more patience out the hole will help. Wait until the backswing is fully complete and the bell is about to passively pendulum forward before initiating your hip drive.
Have you spent a lot of time with the touch the wall drill and especially the KB deadlift?
To get the feel for sitting back into the hinge, use the touch-the-wall drill:
--Stand facing away from a wall with your heels a foot or so from the wall.
--Sit back into your hinge and touch your butt to the wall.
--Move a little further away from the wall and repeat.
As you move away from the wall, if you sink your hips straight down (squat) you won't reach back to the wall. If keep your legs too straight, you also won't reach back to the wall.
To get a feel for coordinating the timing of the hips and shoulders so everything starts and finishes together at the endpoints of the standing plank and full hinge, use the KB DL:
--Do sets of 20. You can use one bell or two, and one or two arms on one bell, but do sets of 20. I found that the second 10 in a 20 rep set is where my brain and body really started to feel things and figure things out, but you have to do the first 10 to get there.
--Start with the bell(s) back between your feet toward your heels, keep the arms in tight to the body and try to lower the weight to the same spot. The bells will want to drift forward, so you will have to really sit back and keep your arms in tight. Avoid any tendency to get squatty to keep the bell back. Do it by sitting back; your hips will move down as well as back, but keep the hips above the knees and the shins relatively vertical.
--Focus on your timing. The tendency is to start lowering the bells by sitting back as far as possible and THEN continue to fold forward and bend the knees to reach the floor. So it ends up being a two-part movement. See if you can adjust your timing so that your hips, knees, shoulders, and the bell all start and finish together. This tends to happen naturally on the positive part of the movement, so see if you can reverse engineer the timing of the positive and apply it to lowering the bell.
--Focus on your rooting. Keep your weight evenly balanced over your whole foot and try to maintain that same weight distribution throughout the range of motion. I visualize the soles of my feet extending straight down into the ground, as if I were wearing tall flat platform shoes that are sunk below the surface of the ground. In the actual ballistic swing, just focusing on maintaining even balance over the whole foot throughout the whole movement can often automatically fix a lot of timing problems.
--Get a feel for the endpoints of the lift (the zipped up standing plank and the deep hinge) and use those as targets in your swing. When I was recovering from a shoulder surgery (rotator cuff repair after injuring it playing basketball), I did a LOT of KB deadlifts because I could do them long before I could do ballistics, and I developed a new appreciation for them. I found that they were actually very valuable in helping refine my hinge pattern, even after deadlifting and doing KB ballistics for decades, and had a lot of carry over to my KB ballistics when I returned to them. By grooving the pattern at slow speed, I could naturally and smoothly transition from plank to hinge and back at ballistic speed, with everything starting and finishing together.