Basically tests like that are "nothing". So the answers [drawings] people provide, when taking such a test, are "clues" as to what is inside the person's head.
Something similar might be to walk up to someone and say "Tell me about yourself". All sorts of ways a person could answer a question like that. Anything from wanting to tell you things about themselves, to eventually go on a date with the questioner, to wanting to start a fight with the questioner for asking!
Or like "Ink Blot" tests...
Rorschach test - Wikipedia, the free encyclopedia
Or "Free Association"...
http://en.wikipedia.org/wiki/Free_as...on_(psychology)
More reading - Projection....
Projective test - Wikipedia, the free encyclopedia
Psychological projection - Wikipedia, the free encyclopedia