Bottom feeders differ from the usual PV in that they have a small, usually vinyl plastic, bottle below the atty/carto and in contact with the atty/carto via some sort of feed
device. The feed
device usually is a tube extending nearly to the bottom of the bottle and connecting with the atty/carto via the center hole usually with a large-bore needle.
Rather than use a syringe or drip
down the sides from the top of the atty/carto, periodically you "squonk" (squeeze) the bottle to force
e-liquid up into the atty/carto. Some have you rejuice the atty/carto by inverting the whole device while you squonk the bottle or push the feed button.
They're superior to tanks insofar as they (usually) don't leak, and you don't have to carefully watch the level. They're superior to attys/cartos insofar as you fill the bottle with several milliliters of e-liquid at a time and don't have to constantly top it off with more e-liquid.
Hope this helps!