What is a bottomless portafilter and why do enthusiasts use them?

A bottomless (naked) portafilter has no spout — espresso flows directly from the basket bottom and falls into your cup. This lets you see exactly how the shot extracts: even extraction looks like a golden, even stream from across the whole basket. Channeling shows up as spraying, spurting, or blonde-ing on one side. Bottomless portafilters are a diagnostic tool for improving technique — issues visible in extraction often can't be detected any other way.