On a reliable horse, maybe a schoolie, or on your horse on a lunge line, tie a knot in your reins and ride in two point at the trot and canter doing the classic indepenent seat exercises - hands on your hips, hands out stretched, hands on your helmet, then add a couple of variations. Stretch your hand and arm forward to the horse's ears and see how far you can go before your shoulder starts to tip down. Also stretch your hand and arm toward the bit in a mock auto release and experiment to see when your shoulder starts to drop. As an experiment, hold your breath and tense your arm and repeat those exercises, pay attention to how stiffening your arm makes your upper body follow your hand and fall forward.
Once you've had some fun with this; set up a simple gymnastic with distances that are comfortable for your horse - trot poles to a crossrail, one stride to a vertical, one stride to an oxer. Keep it small, about 2' and ride through the whole thing a couple of times until you have a nice rhythm, then knot your reins and repeat all of the above exercises through the gymnastic, including the one where you hold your breath and stiffen your arm so you can feel what it's like when your body follows your arm. Use this exercise to experiment with different releases and a whole range of arm motion.
On another day, start through the gymnastic without reins, then pick up your reins and practice your releases. You'll probaby see a little regression and will go back to holding/tensing somewhat, but now you should be able to *feel* it happen. If you need to, go back to hands on hips or hands outstretched to loosen up again.
Finally, your bank photo: Some pivoting on the knee, crotch in front of the saddle, upper body too far forward. Since you have a lot of slack in the rein; your release isn't restrictive; but you can see that planting your hands down by the withers has made your elbows pop out.
Here's some advice I got years ago about riding banks. Ride them as if they where a regular fence, but at twice the height. So if it's a 18" bank, the horse's jumping effort to bring their hind end up onto the bank is the same as for a 3' simple fence. So your position should be similiar to what it should be for a 3' simple fence, but with a more open knee angle/you out of the tack more (so the saddle doesn't come up and bump you as the horse brings his hind end up on the bank.) The photo looks like you know you should be out of the tack more since it's a bank, but you've gone forward out of the tack instead of just opening your knee angle and going up. I'll try and find you a good bank photo.
Hope that made sense. I need another cup of coffee to explain better.