It is not a paranormal concept. It is based on psychology. However, many of those that are into it are also into palm reading, which is a paranormal concept.
The idea behind it is that your personality shows out in all your actions. The way you walk says a lot about you, as does the door, coat hanger, pen color, paper size or seat row, that you choose. Something as open as a handwriting has so much room for your personality to make its path.