I read an article by Dr. Allan Schore a while ago (that I have never been able to find again, darn it!) that suggested that the normal reflective process in therapy (sympathetic look, sympathetic voice, reflective body positioning) was one of the reasons talking therapy works.
In infancy, we are unable to fully process emotional content. Instead of doing it ‘ourselves’, our caregivers help out. They serve as functional extensions of our brains, by their response. An in-tune, reflected response stops the stress cycle. For example, baby is scared by a new event. Stress hormones circulate as baby startles or starts to cry. Parent (in tune) makes reassuring noises, which generally mirror the affect of the infant, facially, gesture-wise, and in voice tone, BUT moderate the response toward a more positive affect (that is, not as scared/sad/angry as the infant, but relevant to it). The infant responds on a neurological level, stress hormones drop (though some of the ‘upset’ behaviors may remain, depending on the situation - such as crying from pain will remain, but the distress portion of it drops out of the system). Same process for happy events - caregivers constantly moderate affect ‘upwards’, giving cues to the baby about how to respond to different events. By being in tune, the infant learns that this feeling of ‘sad’ matches that feeling expressed by the parent. It is the basis of understanding, in the fundamental sense - children whose caregivers do not function normally in their responses have difficulty identifying even ‘universal’ expressions of emotions.
So, how does this relate to therapy?
You’ve got a stress, related to an event. The stress is in some cases chemically interfering with your function (triggering dissociation, overwhelming anger or fear, etc.). Too high a cortisol level, and all sorts of normal brain function start getting messed with, including formation/storage of memory (hence memory mishaps from abusive childhoods). By bringing up the stressor in a ‘safe’ environment, you have the opportunity to have someone serve as a controll portion of your brain, resetting your stress response by responding in a moderating fashion to your stress.
For example, you talk about an upsetting event where you were afraid. The therapist makes comforting sounds, in tune with your response but not as severe (that is, upwards affect, but related/reflected). Their body posture, facial expression, etc., also line up with what your brain, in a neurobiological sense, requires in order to re-set the stress reaction to a lower level. You may also talk about ways to recognize your own feelings, the feelings of others, and manage similar events in better ways. Those are structural/behavioral benefits beyond the neurological one.
For me, just the talking was the biggest thing. All my therapy involved me leading, me teaching myself, me noticing what I was doing and coming up with solutions myself 90% of the time. But I needed that reflection of another face/body/voice to release the chemical stress reaction to the event.
Therapy helped me LOADS. The hardest part for me was locating the actual source of the stress, so that I could release that portion of it.
For more about how the basic function works in infancy, try here, Normal Attachment and Mental Health - it is dense, and hard to slog through all the jargon, but it is really fascinating. There’s a second page (linked at the bottom) that deals with the implications of abnormal attachment process and subsequent mental health. There is another, more readable (and more condensed) version of the same info here. Unfortunately, I have never again found the article with the comment on how this relates to therapy. But it does make a huge amount of sense (even if a lot of therapists might be somewhat offended at the implications).