I think SpookyTooth has a pretty good explanation here;
Spooky Tooth Cycles - Motorized Bicycles: Gas Powered and Electric Bikes - Expansion Chamber Exhaust
Or for a more comprehensive explanation here's the wiki for expansion chambers;
Expansion chamber - Wikipedia, the free encyclopedia
What it amounts to is those hot rods are four strokes, the "wave" created by a properly placed expansion chamber on a 2 stroke helps mimic the effects of valving.
Hope this helps