One of the more interesting things I've read about cats is that the tail swishing they do isn't - as some think - a sign of anger but rather indecision. You see it when it's a fight/flight situation but also if there's an open door on a rainy day; the animal wants to be out but also doesn't want to be wet.

The interesting supposition was that it was a physical manifestation that reflected an emotional state. That is to say a moment in balance - and for cats balance is corrected with their tails. I don't know if it's true but I kind of want it to be.