A transmission line features a complicated exit path filled with absorbent material that will ideally absorb all the back radiation that enters it as opposed to having the air rush out as it does in an ordinary ported design. The purpose is the same in both cases, to simulate the effect of a much larger enclosure by reducing the back pressure created by the woofer. They can work very well if properly designed but like everything else has its virtues and defects. I have had them but long ago.
