Brewing "bottomless" or "naked" refers to brewing without a spout, so that you can observe an unobstructed view of the filter screen where the extraction happens.
This is vital information for determining the evenness of extraction across the entire coffee bed. If you observe regions that are significantly lighter or darker than the average, you have areas of over and under extraction relative to the whole. This suggests that there may have been issues with your distribution or tamping for that given shot, and that brew water has flowed more easily through some parts of the coffee bed than others due to channels opening up under pressure.
When channels open up, they allow water to flow faster in a localized area. These higher velocity streams, relative to the average flow through the cake, usually result in sprays or sprites emanating from the underside of the filter screen. If you have the spout attached, you won’t notice these forming. Brewing bottomless, however, you will notice these very quickly as they spray your counter and anything within reach. While it may make a small mess on your counter, it will allow you to dial-in your next shot for a better extraction!
We recommend utilizing a small compact mirror, such as our Shot Mirror, to allow you to more easily observe your bottomless extraction as you pull your shot.
The Classic, Signature and PRO 2 user has the option to brew “naked” or “bottomless” by removing the included spout. The Flair 58/58x arrives "bottomless."