What is meant by subwoofer room pressurization?

I've recently been researching some subwoofers and people frequently mention the fact that they pressurize the room, supposedly resulting in better sound, not just better bass. Could somebody please explain this to me? I've searched other threads, but to no avail.

Thanks in advance,

It's one of those things that you'll understand once you experience it. It's a sense that the whole room has become an enclosure for your bass drivers, and you're sitting inside the woofer's cabinet.
Good answer Bondmanp. I'd add that you get the sense also that the air around you has become vibrant/pulsing/throbbing with energy.
Bondmanp is right. If you have a good sub that can dig deep and you're minding your own business watching a movie AND all of a sudden your clothes move before you hear anything, you have experienced room pressurization.

But don't make a buying decision based on that factor. There aren't that many movies that "stimulate" you in that way. Buy one that is clean and defined down to 20hz or so and you'll be happy.

I have an SVS PB Ultra 13 and I don't run it "hot". As much as I like bass material, I don't look for it when it's just not there on the recording.

I think of it mostly as an HT phenomenon associated with explosions, etc. where you actually feel a pressure wave. A related but different phenomenon occurs with low pedal notes of a pipe organ, where you may feel more than hear them, just as at an organ recital.

I use a pair of Velodyne HGS-15s with an SMS-1. I set the levesl so the subs are not observable sources of sound.

What's the point of subs if they are set so they are not observable sources of sound? I set my subs so they blend seamlessly with my main speakers.

Not observable and seamless mean the same thing to me. I aim for a soundstage in which the listener is unaware of any speaker, just fine imaging within the soundstage.

Some say that when a wave is unable to unfold that it 'feeds back' on itself. Thus pressurising the rooms' volumn. I think the frequency is about 150 hz for a big room to 350 hz for a small room.
OK, I don't care about movies; I thought it might be an important concept for stereo listening.

Thanks all.
I purchased two 15" subs. I first placed both them on either side of my equipment cabinet facing the audience. I was very disappointed in the impact, so I started reading the sub manual and tried different placement layouts. To get the pressurization effect you're talking about I placed one in the front facing the audience and the other at the back on the opposite side of the room. Now I have bass air pressure coming together right at the point of the seating.

It's a sonic boom... like night and day. The room has become a music chamber and the sound of explosions is unreal. The entire house shakes... the sound is better than a movie theatre.
That happens when the frequency produced by the sub has a longer wavelength than the room can support along any of its dimensions. Usually this occurs below 100Hz (11' wavelength).