Cold Tail What Happens Causes The Tail To Go Limp?

I saw an interesting show on Animal Planet today that referred to a unusual health problem that Yorkshire Terrier dogs sometimes get. It is most often called cold tail but may also be known as wash tail and limber tail.
The condition is marked by the dog's tail going completely limp and the dog trying to bite at the tail as if to rip it completely off.
Cold tail is not considered to be a dangerous and it is only a temporary condition that lasts between 23 days. Because there is some belief that it is associated with swimming in cold water, it has been traditionally called cold tail.
The current belief is that a muscle spasm causes the tail to go limp. Once the spasms stop the tail returns to normal operation.