Having never been taught to round his back, Red's back tends to be pretty "hollow." One of my barnmates told me about a "butt scratch" that triggers a back-lifting reflex. Check out the difference.
We've been doing several of these each day. It is hard work and not one of Red's favorite things.
You can see about where I place my fingers to get this response. I scratch up and down and like magic Red rolls his pelvis and lifts his back. There is a good article on KBR Horse Net, "Care for Horses' Backs". The author calls this reflex the "equine sit up".
Good thing I can totally trust Red to stand while I touch him anywhere.