A bottom feeder injects
juice into the atomizer, it's a way to drip without dripping
Your basic bottom feeder design has a bottle inside the mod that is attached to the 510 connector. When you squeeze the bottle it pushes
juice up through the connector center post and into the atomizer. The Reo and Vmod are two of the most common examples. If you look up either one you can find some pics and videos that will likely give you a better idea of the concept than my meager description