We talk about it where appropriate. My T is exceptionally self-reflective and I'm not sure all therapists would have the capacity to use it in the way he does.
It normally comes up if I perceive a change in the way he is or if he says something which strikes me as hurtful/odd/non-empathic. I usually tell him how I feel about what he has said or how he is being. Then he will think about what I said and if there's a countertransference response there he'll often tell me. It confirms my perceptions and had allowed me to trust my perceptions a lot more in general. Usually it's something like feeling protective of me (sometimes due to paternal countertransference), or struggling with some aspect of my process. He doesn't usually share how it relates to his 'stuff', he deals with that in his own time, as he should.