I have been trying to make sense of the Jennings book myself. If I am understanding the part about "mean port opening". The port(s) are essentially smaller than they would appear due to they are only fully open at TDC and BDC. So, in order to compensate for this, the port timing or port area has to be altered for the port(s) to function efficiently for the intended RPM and displacement of the engine.

I may be wrong, but I hope I'm at least missing closer.