Works on a similar principal as putting a subwoofer in the boot of your car (if you've got one try this - sit in the front seat and try the boot door both and closed you'll hear quite a difference) it's a bit like the way bass reflex ports work aswell (not exactly) basically the lower end of the notes you play are reflected back towards you and create a seamingly bassier response as the frequcies aren't dissipated out of the back of the cabinet. Type your question into a search engine I'm sure you'll find plenty of other forums giving you better answers.
