There is a powerful sense of movement in this painting. It's not only because the horse and rider are in an action pose. It's due to the treatment of the composition itself. The tree is already falling behind. The cloak and tail are trailing in the created breeze. There is a sense of urgency in the darkening sky. The tree seems to lean forward trying to catch the rider off of his mount.
I see the animal being held back a little as well with those tight reins. Is he trying to turn or is something below the next rise that he doesn't want to run headlong in to?
The colors are fantastic. The muscles on the horse are accurate and show the movement well. I love the tiny details like the cloak in tatters and the bushes in the for-ground. Nothing was an afterthought in this painting. It's simply stunning!