I see, it seems the author fall into the misconception of "line of eyesight" thing. FYI it's not true for master swordsman, line of eyesight can be deceived and not change at all, heck in fast paced battle if your eyes move first than your body you most likely get stabbed first, the right way is looking around their chest where you actually can precieved what motion he gonna do, by watching his muscle, joint and shoulder movement.
NB, this is shared by my acquaintance, a former HEMA Instructure.