My therapist says he uses his countertransference to move the therapy.

If he feels mad, then I am likely mad.
If he feels sad around me, then I am likely depressed.
If he feels frustrated, then he knows I'm likely to be as well.

In fact, a physician friend of mine (a hematologist no less!) says the best way to tell if a patient is depressed is if you feel depressed around them.

In essence, it seems the well trained therapist is able to use their own feelings to uncover and explore how the client is feeling.

But, yes, they have to be able to look outside themselves and rationally evaluate what is their "own" baggage, and what is actually in the room, coming from the client. It is a skill developed in their training.

That's why I think understanding how therapists deal with countertransference may fail to help with transference.

We're in therapy to sort out what is our own baggage versus what is actually going on and to develop the skills to know the difference.

Of course, that doesn't mean transference is useless in therapy, in fact, just the opposite. I think it can move the therapy as well, but it is not solely up to the client to "handle" it.

Letting your therapist know how you feel and exploring why you feel that way can be very important in developing an understanding of yourself and how you interact with others.

Just my two cents.