I think, without being an expert, that there are different things that make it work. A friend of mine says that for him, what works is when the therapist catches him out with contradictory or illogical thoughts, which makes him embarrassed enough to change his thinking. For me, I think it's more about changing neural pathways by rehashing and repeating the same subjects from new angles. (Thoughts and feelings are all about chemicals and hormones and electric impulses in the brain, after all.)

It is a slow process though.