It's more about the resonance set up by the port or passive radiator interacting with the driver and the box. You can think of them as low frequency amplifiers for your main driver. As your driver starts to roll off on the low end the resonator starts to pick up giving you extended low end. The trick is to tune the resonator so that it ramps up at the right point where you driver ramps down so you don't end up with humps in your frequency response.

Of course some people like those humps in the frequency response because it maske your sub sound louder. for example Outlaw has the LFM-1 EX with two ports: plug one port and you get "Bass Extension". Leave them both open you get "Max Output". Opening both ports moves the resonance higher so that it adds to the main driver instead of complementing it and extending the low end.

Sealed subwoofers do not rely on resonance to boost their output. They are typically considered the most "musical" (Jimna can tell you what that means. I just know how to build the darn things:) of the subwoofer designs because the fequency responses can be kept pretty flat. However they tend to be big and require a lot of power.

I don't understand H-PAS technology and how that relates to the above so I'll just end here.