it "tricks" the sub into thinking its in a larger box therefor sounding better/louder its great when space is an issue just build a smaller box and stick some of that in there-- also it helps as kindof a dampining factor so you don't get that dreaded rattling of your car
WallyWorld one of the best stores (price) even tho they pay there "slaves horrible" just go into the arts n crafts part-- its basically just pillow stuffing