2 korrents from this piece
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Their wordsI now suspect that adding a source of hydraulic resistance under the puck that is non-negligible compared with the espresso puck itself is desirable to reduce the impact of channels because it slows down the flow of water where it would tend to become otherwise too fast and as a consequence less even across the surface of the coffee bed.
Their wordsflat burrs tend to send the coffee preferentially on one side of the portafilter (to the right side with the design of EG-1), an effect probably worsened by the use of high rpm.