MTK,

First, the axis in the figure is Time on the vertical axis and lenght on the horizontal axis.

The exhaust pulse (A) is seen to leave the port and to propagate toward the atmosphere, primarily sending suction refrelections (B) back toward the engine from the diffuser , and compression wave reflections (C) from the tail nozzle. The density of the shading(difficult to see in copies)is intended to represent superposision pressure, where the more dense the shading the higher pressure. In an optimized design the primary reflection behavior ensures that the peak of the suction reflection returnes to the exhaust port around the bdc period to aid the scavenging and induction process and the plugging pulse returnes to the exhaust port in time to hold the in the fresh charge and enhance the charging effiency, before the exhaust port actually closees.

The picture of time and distance is one which will remain similar in all engine speed, therefore it is the crankshaft timing points which move on the vertical axis as a linear function of the engine rotation rate.
As commercial tuned pipes do not (usually) have a diffuser build in, the comparison will be between a straight pipe and a purpose built pipe where the engine design parameters are reflected in a purpose build tuned pipe.

One method available to extend that band of speed downward over which the plugging pulse would provide a significant gas trapping action is for the pipe to remain the same lenght dimention physically but for the exhaust opening port to be redused at low engine speeds by a valving mechanism.

Well this is brief but hope it contributes.