I'm guessing they were referring to phase velocity: https://en.wikipedia.org/wiki/Phase_velocity https://en.wikipedia.org/wiki/Phase_velocity You can see the

I'm guessing they were referring to phase velocity: https://en.wikipedia.org/wiki/Phase_velocity https://en.wikipedia.org/wiki/Phase_velocity You can see the effect shown in the first animation of this article if you keep track of the wave peaks in the wake of a boat. They start at the back of the group of waves, move through it, and fade away as they reach the front. https://www.youtube.com/watch?v=lWi_KpBy8kU https://www.youtube.com/watch?v=lWi_KpBy8kU