They are also called velocity stacks, as seen here on a 12 cylinder Ferrari GTO.
(Please to be not stepping in my drool puddle thank you)
See also, Venturi
http://en.wikipedia.org/wiki/Venturi_effectWhen the air gets squeezed down through the smaller opening, the pressure drops, but the velocity increases.
True "tuned exhaust" uses the same principle. The engineer creates a low pressure zone in the exhaust pipe that helps move exhaust out faster than natural aspiration can.
I recall some years ago a wood cutter's competition where all the competitors were using tuned exhaust on their 2 cycle chain saws. The saws were 2 feet long, and the exhaust pipes were 3' long. They had the wicks turned up to around 10 or 12,000 RPM. Very loud, very fast, very cool.
A more modern competition saw with tuned exhaust
http://barkhamstedhistory.org/images/Mike%20and%20super%20saw.jpg
