Therapy is as much art as it is science, so you could get almost as many answers to how therapy works as therapists you ask. People, relationships and emotional healing are so complicated that we therapists tend to resort to metaphors and generalities to try to explain what we do. I'm a poet; I have nothing against metaphors, but I also am very pragmatic and like explanations to be clear and concrete. Because therapy is a place we invest our privacy, our money, our mental health, our trust--very dear treasures, we need reassurance that we are making a good investment, and yet it is so hard to get this reassurance when we are investing in something that is different for each individual client and therapist.
I have thought about this over the years, beginning with school, when my frustration with studying animal research and lots of contradictory theories led me to the mantra, "just tell me how to heal people!" "Don't tell me what some man 50 years ago thought about the nature of human development; tell me what to say and do with the suffering person!" Well, 25 years later, I have a firm grasp on what heals people, and it is still different with everyone, and it is still an art. Here's what I've been able to isolate to almost a science:
1. Talk It never ceases to amaze me how healing just talking about the thoughts and feelings causing emotional pain and confusion can be. For many people, just doing this during their first session begins to provide relief. Saying it out loud, hearing themselves say it, and having a witness who understands their experience and can communicate that understanding with compassion goes a long way toward people feeling better.
2. Emotional Release Sometimes just talking helps, but often talking with a chance to release the emotional tension that came with whatever is troubliing the person is even more healing. We are meant to cry, to yell, to release physically from our bodies the stress chemicals that accumulate when we experience a loss or trauma, or even a joyful experience. Most people experience relief and even a new perspective after talking about a problem while expressing the feelings that go with it.
3. Insight Sometimes talking and expressing the feelings still isn't enough. Sometimes having someone else point out a pattern, see a meaning or motivation behind what you do, show you how one piece of your life connects to another is essential to relief. For example, a woman is angry about how her partner is treating her and talks about it, expresses the anger, and then comes to see that how he is treating her is in essence the same way her mother treated her. She may begin to feel less like an unlucky victim, and more like a person trying to resolve an old relationship by choosing a similar new one, and finding the outcome to be the same. This is the beginning of her being able to understand why she gets into relationships with hurtful people and empowers her to make a conscious decision to be with someone who treats her better.
4. Relationship Sometimes even talking, emotional expression and insight don't change people's lives for the better. This is the place where people want to try to say affirmations, or change their thoughts to more self-loving, hopeful ones, and find that either impossible or unhelpful. At this point, people need something even deeper. Being in therapy offers a unique opportunity to heal within a relationship. Because It is people we have some sort of relationship with who hurt us when we are kids, and it also generally needs to be within some sort of relationship that we heal that hurt.
However our parents relate to us, we imitate,or carry as a template in our minds, defining it as love, and looking for love that matches this definition. So when we relate to people, especially the more intimately, and the longer we do, we bring with us a set of patterns of relating. We may relate to people suspiciously, angrily, timidly, lovingly, trying to please them,etc. We relate partly like our parents, siblings or other important people related to us when we were young and trying to figure out the world. We may also relate partly as a reaction to these people. But whatever patterns of relating we have are actually memories--windows into past relationships, and how those relationships affected and formed us.
Being in therapy can be a very intimate experience of relating. It has all kinds of boundaries so that it doesn't become just one more hurtful personal relationship, but it can still, within the boundaries, be very deep and intimate. Relating on that level, reveals those old relationship patterns and memories. A skilled therapist can use that information to help you understand what you are doing, where it came from, why you had to learn to do it that way to survive, and how it's affecting your life now (insight). The therapist gets a deep sense of what it was like to be you as a child in these original relationships, and can communicate for you what was probably too scary or too confusing for you to articulate at the time (talk). This can lead to deep, old emotional expression of what it was like to be you as a kid in relationships with the people around you (emotional release). So much can heal within this relationship. On top of talk, insight and emotional release, a healing relationship with a therapist gives you a chance to learn how to love and discipline and teach and encourage yourself, by imitating how your therapist is with you. We have special "mirror neurons" specifically oriented toward imitation, whose purpose it to imitate our parents so that we will learn how to survive in the world. If we imitate abuse, we don't treat ourselves well. Imitating a therapist's compassion, empathy, forgiveness, gentle but firm boundaries, patience, kindness, perspective, etc eventually allows us to treat ourselves that way--overwrites the imprint of our parents' behavior. The relationship with your therapist can also be a source of motivation to do things you couldn't get yourself to do on your own. It is encouraging and rewarding to have a relationship that creates a strong, loving foundation from which you can explore new behaviors, thoughts, feelings, etc.
5. Integration We all have different parts of ourselves. For example, we may have a child part who wants to eat Cocoa Puffs for breakfast every morning, and an adult self who wants to eat something that will make you feel better a couple hours later. We may have a teenager who loses her temper and wants to run away, and another part that wants the teenager to shut up and not cause trouble. The more the parts are in conflict, and wanting to act independently, the more mad we are at ourselves, and the more we are confused why we can't change behavior we know is causing us problems. The process of giving each part a chance to express itself, of treating each with respect and understanding and gratitude for it's well-meaning contribution can integrate the parts into one coherent whole, and allow you to do what you decide is best for you. Sometimes EMDR is necessary to create this integration, but it generally happens over time in therapy.
6. Behavior Change It is often only after doing 1-5 that people are able to do what they often know they need to do when they come to therapy, but can't get themselves to do. After integration, deciding what behavior works best for who you are and want to be and acting on it is usually pretty easy. I find that it is at the end of therapy that people can change behavior, and thought patterns too. I don't believe cognitive/behavioral treatment (which is emphasized in research and popular media) is useful at the beginning for most people who come to therapy. At the end, it happens naturally.
So maybe this makes clearer what happens in therapy, how it helps people get what they want, and why it's so complicated. I'm sure I haven't clarified everything, by any means, so if you have questions, I encourage you to post them on the forum, and I'll try to clarify even further. That way, everyone can benefit from your question.
Plus One I haven't covered EMDR here, which is another very important way people can heal. For more about that, look at my EMDR FAQ page.