Forced perspective is a film making technique employed to make larger objects appear smaller to the viewer or vice versa, depending on their relationship to the camera and each other. Forced perspective creates an optical illusion, used primarily to make objects appear far away when set space is limited. Movies (especially B-movies) in the 50's and 60's are notorious for obvious and poorly done forced perspective angles.
No real rules, but photos that do not demonstrate forced perspective will most likely be removed from the pool. Photos that are FP photos but do not show obvious forced perspective should--although not absolutely necessary--add how technique was applied in either the description or title.