Mosr often, they are called pressurized portafilters. They are found on a lot of the lower end pump machines. Saeco, Starbuck's Barista (made by Saeco), Solis SL70/90 and others.
What this does is: as pressure in the portafilter builds and when it reaches a predetermined pressure, a spring in the portafilter opens and forces the espresso out a pin hole(s) opening. This forces foaming of the espresso and they call it crema enhancer. I've used both and personally think the "enhanced" crema takes away from the taste of the espresso.
When I first got my Isomac Tea, I used fresh, home roast beans and got more crema from it than I ever did with enhanced types - even using the same fresh/home roasted beans. Of course, the shot was much better and really isn't fair to compare apples to oranges, or in this case, lower end machines to high end machines. It is however, my experiences so I pass them on for whatever they are worth.
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