I've recently had a bit of a discussion with a friend about tail-twitching. Specifically, what does it mean when a horse twitches his tail while performing or executing a movement? I've seen this mostly in dressage-type work: piaffes, collected canters, pirouettes, etc. I thought it meant the horse had either been trained with somewhat aversive methods or that the horse wasn't completely mentally OK with the work, but my friend didn't think so. So ... what does tail-twitching mean?
Here is the video she showed me that initially sparked the conversation. Even my non-horsey sister commented that it looked like the horse was uncomfortable.
Also, was that a clicker he was using in the video? If so, can someone please explain exactly what the trainer was marking with the clicker? It sounded like a clicker, but it seems to be very different from the way dog and cat trainers use it, which is my only experience with a clicker.