Part of it is a dog's natural tail carriage, but it's mostly body language. If a dog is very interested in something, in a positive or negative way, the tail goes up.
Thor has a nubbin too. I also grow the hair out long, and once I did, I found even the nubbin to be wonderfully expressive too. He wags it a lot more than I realized. It's very cute. I'd still love for him to have a full one, though.
Thor also uses his to prevent "back door access" at the vet.
