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David Vizard takes about both of these things and you should go re read it again.
#1 SCAVENGING
this is merge collector(remember the cars merging onto a freeway?)job to help keep the exhaust flowing. One high pulse (exhaust valve just opened) and gas travaling at high speed exit into the collector were another pulse is just about done(exhaust valve almost closed, intake valve starting to open) High speed gas keep the slowing flow going just a little longer causing the exhaust to pull on the intake charge for that overlap period. HOW EVER! If there is excessive back pressure, the gas flow stops! and then back up into the combustion chamber through the exhaust before it can close and stop this. Now you have inert or unburnable air in the combustion chamber. Now less burnable air/fuel can get in(vary low power stroke.(you can have no back pressure problems and have the "wrong cam" for you application and still get this same situation. this is called "REVERSION" OR EXHAUST GAS REVERSION!
Pulse/soundwave tuning
Now turned length header can be made that when the exhaust valve close there is a pulse or sound way that runs to the end of the pipe, bounces back to the exhaust valve and then back to the end of the pipe. IF YOU GET EVERYTHING JUST RIGHT, AND THE EXHAUST VALVE OPEN JUST AS THAT WAVE LEAVES THE VALVE HEADING TO THE END OF THE PIPE/HEADER YOU GET A SUCTION EFFECT TO HELP PULL ON IT.
NOTE: When Mr Vizard is talking about this he is NOT talking about the Exhaust pulse/wave! He is talking about the intake wave that help push more air then the piston can pull.
NOW! Go reread Mr Vizards article before you try and find fault in my explanation!
I know you won't, but it was worth a try:poke::D
He must have skipped that section... 
