In case you have not seen it

http://www.advancedpilot.com/downloads/prep.pdf

Hmm, what should I say..

I am not very impressed with that explaination.

Firstly the pressure-term "inches of mercury". Often misunderstood, and he should have explained that. How can pressure be related to a distance of metal? It does not make sense at all, unless you talk aviation oldschool term. Pressure is force/area.

Suction is Bullsh.. He should have emphasized more about absoulte pressure. So, ambient ISA pressure is one thing, NOT to be confused Zero pressure. That is not the reference. Suction is a "negative value", but it is not possible to get pressure below Zero(absolute).

Short and sweet:
The MAP gauge gives you an indirect (not linear) indication of how much of the available power you are pulling at that specific RPM.

There should have been another dial underlying the MAP showing outside absolute pressure (a kind of altimeter) to show maximum available power at your current altitude.

It would have made much more sense to scale it in % instead of the nonsense inches metal.

The biggest problem I see with the document is that he keeps talking about sucking the air in. This is simply incorrect. A brief visit to his local college physics department will put him straight. Air flows from high pressure areas to low pressure areas and is pushed by the pressure differential.
If the author were to start using that tidbit of physics his document would make much more sense. Particularly the explanations of why manifold pressure changes with rpm while holding the throttle plate steady.
There are a number of other errors, large and small. Not very impressive.
Had a look at their website. For what they portray themselves as, this error is pretty unforgivably poor. Guess it goes under the heading: "bravado and volume must mean it is true". That or "in the land of the blind one-eye is king"