The blurry effect is actually the opposite image but blurred off. the sharp one is the opposite of the bluured one.
Eg. the blur imeage of the calm face is behind the sharp image of the angry face. and becos we can only see the blur things from far (theory of short-sightedness), the blurr image looked clearer, hence replacing the sharp image....
dunno whether did i explained clearly?
HAHA