Actually I think I get what he is after, and it is a little interesting. As wave moves the wave length must change in exact proportion to speed so if the waves slow down (for example due to entering shallow water) then the wave length will be proportionately shorter. The result is that number of waves that pass by a fixed point during a time interval stays constant, which is not intuitive(but mathematically quite trivial). If the waves slow down 2X the wavelength is 1/2 what it was and thus the waves/sec is constant.

Duncan