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 blonde streaks in the stream of espresso; sometimes the puck will even have pencil lead-sized holes where channeling occurred. This bottomless portafilter is specifically designed to fit with Nuova Simonelli espresso machines with a 58mm group head such as the Oscar & Musica. It is designed similarly to the portafilters that come with the machines including 'wings' at an angle, just without the spouts or bottom to the holder. The sturdy plastic handle can be removed and swapped if you so choose.