Psycho-dynamic play therapy, when performed by a psychologist, works in a number of ways:
Corrective Experience: The very relationship of the child to the therapist is used as a tool that gives the child a "corrective experience."
True Self: Allowing of the emergence of "true self" for children who feel a need to please others, or in other ways are out of touch with their inner world.
Expression of thoughts and feelings: Kids who are not communicative, learn to express their thoughts and feeling in the safety of the play room.
All of these aspects expand to the rest of the child's life and relationships.