The resistance to water flow, and thus the generation of pressure inside the portafilter, is supposed to be provided by the ground coffee, and the pressure differential as you move down the basket is what extracts the oils and aromas from the coffee. A pressurised basket provides no pressure differential through the basket in this regard, so it becomes somewhat like a drip filter system except under pressure. What comes out of the basket then gets forced through a pinhole normally (which you can often see on the underside) and this froths up the liquid to make it appear like crema.
A day to day example of how this frothing system works is well appreciated by the SNAGs amongst us - a kitchen sink with detergent in the bottom (non-SNAGs insert <bucket with car wash liquid>
. Turn the water on at a moderate rate with no nozzle attached and the water runs into the bucket and not much happens to the car wash liquid. But fit a high pressure nozzle to the hose, and the same flow of water produces copious froth and bubble. This same behaviour is what froths up the relatively crema-less coffee otherwise produced in a pressurised basket and makes it look like the real deal.Also, by creating the pressure by the basket rather than the coffee, it reduces the importance of tamping, which is one factor you cant rely on the average ignorant home user to get right, especially with the tool they normally supply for the job! The users of some of the tampers Ive made for pressurised baskets have reported better results with a proper tamp, but the difference isnt as marked as on an unpressurised basket.
Greg

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