My parent's cat Miss Kitty will quiver her tail like that whenever we start to reach down and pet her. I also saw a female at the shelter I visited Wed. do that when I would reach in to pet her. Maybe it is just an excitement kind of thing. It does look like they are trying to spray.

I've never seen Jazz or Scout do it - they are both girls. Unfortunately the two times I've seen Ripley do it he was backed up to my newly painted wall and was spraying.