I know that when a horse swishes its tail it usually means that they are uncomfortable, etc.
I was watching my friend reining her horse (she was a world champion when she was a 2 year old--shes 18 now). I saw her swish her tail then my friend said, "Did you see her swishing her tail? That means the 'renining side' of her brain is on." I got puzzled but her trainer said that is what reining horses do.
When they swish their tails it means get 'get it'.
Not sure if that made sense. I've seen some more reining videos and during the spin I saw some reining horses swish their tails. However they didn't look irritated. They looked more relaxed, licking lips, etc.
Anyways, I need some reiner's advice!