It means to move your head forward and downward in order to avoid an object or obstruction, it is the same as "keeping your head down". Ducking, in general, is to move into a position that helps you to avoid an object from above. http://4.bp.blogspot.com/-DTYKJpZr0c0/ThtjQ78jb6I/AAAAAAAABLM/srYF-SXbXng/s1600/Keeping+Your+Head+Down+%25281+of+1%2529.jpg