[crazyhorse]

I think I can bring this down to bite size for the less engineering minded among us.

When svxfreq asked for cam timing specs to build headers I knew he was on top of things. (not to say the others who build them aren't)

By tuning the length of the pipes before they merge (primary tubes) you can control the pressure inside the tubes.
Think of the waves mentioned as ripples in a pond. When the ripples from one cylinder meet the ones from another, they cancel each other out. Time them just right, and you can use one wave to suck the exhaust from another cylinder.
Remember, an engine is an air pump, more air through, more power made.
This also means, that the collector (where the primaries merge), must be sized just right too. Get it right, and big power can be made fairly cheaply. Get it wrong, and it'll sound great, but not add much HP.
Yep, it's simplified, but accurate enough for those trying to understand.

In my experience, well tuned headers are quieter than ones that aren't. That may help get you to that point svxfreq.