Pressure Balanced Pex Home Run Manifold?

I was watching one of the varieties of the Holmes On The Range shows. Mike was literally gushing over the Pex home run manifold that had been installed by his plumber. I gotta admit, it was pretty nice looking. Each curved run of Pex was perfectly spaced, with a long, sweeping, graceful layout. Multiple runs went off to the left and the right, laid out perfectly like a pair of angel's wings. :-)
Anyway, Mike repeated multiple times, each time with more passion, that the system was "pressure balanced", saying that that means "if you are taking a shower upstairs and someone flushes a toilet downstairs, the pressure in the shower won't change."
He wasn't talking about a pressure balanced shower valve or consistent temperatures. He specifically said the "pressure will never change."
OK, so how does that work? If multiple fixtures are opened, doesn't the pressure in the system have to drop? It seems to me that the only way for the pressure to never change as each additional fixture is opened is to have the system running at some specific pressure when only one fixture is open and then have it boosted each time another fixture is opened. After all, you can only have so much pressure in the building to begin with - street pressure - unless you boost it with a holding tank and a pump. There was no mention of anything like that being installed.
What am I missing?
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On Fri, 15 Nov 2013 02:36:53 +0000 (UTC), DerbyDad03

That's what they say about pressure balanced shower valves. Some places require them in new homes by code. They basically normally restrict total flow of hot and cold combined, then adjust to whichever side loses pressure. to prevent scalding or a dose of cold water. So since total flow was restricted initially, the actual pressure at the shower nozzle head won't change when the valve allows more hot or cold flow. Actual pressure in the supply piping will always change with different flow rates. Semantic razzle dazzle, since if cold water flow is interrupted in those valves, they shut down. What kind of pressure is that? And some might be designed differently than I've said. I just made it up. This guy seems to know about them. http://ths.gardenweb.com/forums/load/bath/msg112139239337.html
He says they start with 100% flow, then choke down on one side or the other when their pressure is out of balance. Of course that reduces pressure at the shower head, I like my way better.
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The copper pipes used for the manifold were indeed fairly large. Hard to say from just watching the show, but 3/4" - 7/8" is what they looked like.
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On Fri, 15 Nov 2013 02:36:53 +0000 (UTC), DerbyDad03

It's not "balanced", rather since the runs are all "home runs", there is no pressure drop because one branch turns on. This is true, of course, only of the size of the feed can accommodate the sum of the loads. This is also true of multi-drop runs. If you increase the size of the branch at each tap, it works the same way.
The down-side of the home-run strategy is that you can't use one tap to "pre-heat" (or cool) water for another. There is no way to "pre-heat" the water for the dishwasher, for instance. It *will* get cold water to start.
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