[gone-ot]

...an excellent analogy is a 'slinky' sliding through a clear piece of tubing:

The whole length of the slinky represents a "slug" of exhaust gas from one cylinder.

At the moment the slinky leaves the exhaust value, it is tightly compressed together, representing the sudden, high-pressure, explosive out-rush of exhaust gas out/past the exhaust valve into the 'freedom' of the exhaust manifold header pipe. As the slinky moves away from the exhaust valve, it's length expands quickly forward as it quickly moves forward, within our 'glass' pipe, away from the valve and toward the open-end of the exhaust system. The compressed slinky represents the compressed exhaust gas at high-pressure. But, when the slinky finally reaches the open-end of the exhaust pipe, it is no longer under as much pressure, as represented by slinkys' "stretched-out" length.

At the moment the slinky reaches the open end of the exhaust pipe and "pops-out", a "low-pressure" (rarefaction) wave is created/reflected which propogates BACK up the still exiting slinky length (the forward/backward compression of the individual slinky coils)...back toward the exhaust value. When timed correctly this low-pressure reflection arrives back at the just-closing exhaust value and literally "sucks" the last remaining vestages of exhaust gas out of the cylinder...and if there's sufficient valve timing "overlap" it also helps suck "in" the fresh Air/Fuel-mixture from the intake manifold, past the just opening intake valve, helping "suck" more A/F-mixture into the engine than it normally would ingest.

The above process is acoustic (length) tuning, since it's about pressure/rarefaction waves within an air medium (acoustics). Engine speed, exhaust gas temperature and exhaust/intake value timing are the major controllers here.

The physical movement of the gas slug (slinky) down the exhaust pipe is mass (diameter) tuning, since it is the front-to-back passage of the gas slug sliding within the confines of the exhaust pipe area. Engine speed and the squared-ratio of pipe diameter to piston diameter are the major controllers here. A maximum gas movement of ~ 300-350 fps (mach limiting) is optimum.