I have noticed the raised ring on pits for many years, usually occurring when they go on point or really getting into something. My best guess is, it is caused when the tail muscles having mobility are contracted to the point the muscles are adhered to the tailbone structure too tightly to move. Thus, causing the skin to stretch at this furthest mobile point, raising the hair to vertical.
Interestingly, one old line of dogs of Howard Teal had a tendency to throw a screw tail occasionally. It was noted by a few at that time that some of those dogs that did not exhibit a screw tail, would have three raised rings when they went on point.