Izzy holds most of his tension in his back, base of his withers, and poll. Getting him to let go of all of that takes some careful riding. Warming up with just my seat bones and then getting a stretch down before changing the bend definitely helped unlock his back and neck.
To get that last bit to release, Chemaine had me try a new trick. When Izzy just won't get soft, she instructed me to take my inside rein and cross it ever so slightly in front of my pommel and hold it.
You can see why I would need to do that here:
By crossing my inside hand over the pommel, Izzy doesn't have anything to lean on. I become a human side rein. The instant he releases his neck and poll, I can also release the inside rein. Here he is just a few moments later.