Projective tests, including the famous rorschach test or the draw-a-person test are based on the theory of projective hypothesis, which states that

I believe the answer is: If someone is asked to describe a meaningless, ambiguous stimulus, her response will be a projection of her inner thoughts and emotional processes. 

By analyzing this response to ambiguous stimulus, the psychologist could analyze the subconscious mind that the subject have since the response would not be filtered by conscious desire to be seen in a positive light by other people near the subject.