Its basically like a cylinder/chamber in line on the exhaust that filters out certain aspects of the exhaust note, normally paticular engine noises and quietens things down abit Ive been doing research on exhaust tuning for my next project, looking at bore sizes, the effect of heat within the exhaust and different characteristics that different designs and the placement of boxes, CATs and bends will offer NOW THATS GEEKY!!!!!! HAHA (very interesting though) from what ive sussed out, ive gone off custom exhausts unless they are for a custom application (charged/turbo VR etc)