Suppose that there is a pulse moves to the right.
   There is a upward transverse motion along the leading edge of the pulse.
   The transverse motion vanishes at the peak, and there is downward transverse motion along the trailing edge .
Let us consider the superposition of two pulses of identical shape moving relative to each other.

Usage :

Even though the wave is flat
for a brief moment, It is not at rest
(Think about the velocity at each point, y=0 does not mean Vy=0!).

You are welcomed to check out related simulation at [url=]Superposition of two impulses (Gaussian wave) [/url]