Check out this illusion. Watch the spinning disk fall straight down; then shift your gaze to the blue circle, and “watch” the falling disc with your peripheral vision. It will no longer appear to fall straight down. Then, when you turn your eyes to the spinning disc again, its motion seems to abruptly change. This might model the phenomenon that curveballs in baseball “break” suddenly; they perhaps both result from the viewer/batter suddenly shifting his/her gaze from one fixed point to another spinning, dropping object.