Well today was interesting. I had a feeling that Darcy hadn't been truly softening through the ribs because his head level was still above his wither at the end of the lateral drift. I went out with the goal to get a better try with his head lower and even after many attempts and trying lots of different positions, I wasn't getting any better tries. He was quite obviously trying to pick himself up and try for me, as I could see him flexing around me and his stomach tucking up and his back rising, and when I offered the release forward again I got an few offers of vertical flex for the first few steps.
His head was at an acceptable height (only about 10-15cm in total above the level of his wither) and he seemed to be relaxed, so I'm beginning to think that his head level is ok as it is. As an Arabian he has a naturally high head carriage, so his head is pretty low considering.
We did have some good progress with the lateral drift, especially on the right. His hind had been lagging going to the right and Darcy gave me a few tries of bringing his hind end under him in the lateral drift which I released forward on.
I'll keep working on it and releasing with the tries I'm getting, and hopefully we will be ready to try a nice lateral drift at the trot soon.