When the wave travels in the direction the line is moving it has a very fast velocity in the frame of the camera that is equal to the velocity in the loop plus the velocity of the transverse wave.

When the reflected wave returns its velocity going down the line is faster than the line is travelling upward and it appears to be moving down in the frame of the camera.

When it gets near the bottom of the loop the tension in the line is much smaller so its velocity slows down. It is going down the line at about the same speed as the line is moving, and thus it appears to be frozen in the frame of the camera.

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