The “Rule” of Space simply suggests that a photographer taking a picture of a moving object should leave more space in the frame in the direction of travel when composing the shot. Having space for the object to “move into” creates a sense of motion and a stronger composition.
Which of these pictures “feels” more natural or correct? Most conclude the horse positioned in the left side of the frame jumping into space that has been reserved on the right into which the horse may move. The second picture leaves the viewer with an uneasy feeling that the horse and rider are about to crash into the right hand margin.
Remember though, photography “rules” are not absolute. The uneasiness caused by the little girl being positioned so close to the edge of the frame she is moving towards compliments the sense of sadness associated with spilling candy.
The “Rule” of Space also suggest that people should be looking or facing into the frame and not out of it. Advertising or concept art may make use of negative space for the placement of text or a logo. The gaze of the subject forms an invisible line that guides the viewers attention to a certain part of the image.