Simply answer, why the movement direction of point M on the waveform diagram is downward. On the waveform diagram, the vibration direction of a particle refers to its speed direction (except for the highest point and the lowest point, the vibration direction of these two points is its acceleration direction, because its speed is equal to zero). Judgment formula: look at the direction of the wave source. In the figure, the wave propagates to the right. The wave source is on the left. So, m, look to the left. As you can see, the point to the left of point M is below point M. Look to the left, so move down.