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Bottomless portafilters are used as a training tool. It will provide you with immediate feedback and can see if channeling occurs right away.
Channeling is the rapid passage of water through fractures in the coffee puck, which produces a thinner, under-extracted espresso. When it occurs, you'll often see sudden appearances of blond streaks in the stream of espresso; sometimes the puck will even have pencil lead-sized holes where channeling occurred.