Subs put out low frequency bass. Duh. Low bass bounces around the room creating areas of reinforcement and cancellation. You can walk around a room and easily hear places the bass is too loud, or too quiet. Or instead of you moving around you can move the sub around which will move the modes around. This works in 3D. Its just most people aren't into lifting hundred pound subs. Anyway, that's all that's going on. He could get the same results putting the sub somewhere else, it just takes a lot of trial and error to find that place. In other words there's nothing special about the sub being close to or on top of the floorstander. In fact, the best solution for really good bass is a distributed bass array. Four subs placed asymmetrically around the room. Try that and you will never go back.
