Pressure is one of the factors in extraction. More pressure means higher extraction. If the coffee is too coarse for the pressure to build up then having a flow restriction can help build pressure. The VV does it with a small needle valve and spring in the base of the portafilter, most machines do it with a very small hole in the bottom of the basket. As to how it provides a reasonable result, it doesn't really.
Tamping largely only effects the time it takes for the first drop off coffee to appear, after that the shot will run largely the same.
If the shot's Only taking 10s you either need a better grinder or fresher beans.
Tamping largely only effects the time it takes for the first drop off coffee to appear, after that the shot will run largely the same.
If the shot's Only taking 10s you either need a better grinder or fresher beans.

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