There are a few different tail cues that might be interesting for your horse to learn.
The first one is to resist (brace against) when you pull on his tail. This is good for body work. The second is to give to the pull, for backing up.
The following pictures show the horse bracing against the request (pull) and giving to the request. For the brace, we ask the horse by a harder pull, and brace within your own body. For the "give" for backing up, we ask lighter and partner it up with a "kiss" (which means to the horse to move your feet).