I finally found a medical description that I can understand…
Horse Veterinarian: Doctor.A
“…sidewinders can be due to a large variety of causes, but the syndrome is usually fairly consistently defined as a neurologic deficit in the hind end including a difficulty knowing where their hind end is, AND/OR severe weakness. Often times there is also muscle wasting of the hind end. The front end is most often normal, as is their mentation (mental state) and personality.
When standing, they will often lean to one side, and when walking, they will tend to drift toward the side they are leaning to. When you pull their tail, there is little to no resistance, and they are easy to “knock off their feet” if you pull too hard.
They will often respond well to a course of antiinflammatories, but this response is transient, and will get harder to maintain with the same dose of antinflammatories. Often, the course of this condition will result in a down horse due to the weakness in the hind end. This can occur over months to years.”
One thing that was clear to me was the difference between the onset of Sidewinders and EPM. I had a young mare who developed EPM some years ago, and the symptoms “crept” into our life. She seemed fine at first. Then she got more mellow. (She was normally opinionated.) Then she started to have the balance issues. With my older thoroughbred, he went from normal to leaning precariously in a few hours.