Reverse Bowden

 Really good description of what a 'reverse bowden' setup is for a direct drive printer. - https://forum.prusa3d.com/forum/english-forum-general-discussion-announcements-and-releases/reverse-bowden/ 

To understand what a reverse bowden is, first you need to understand what a regular bowden and direct drive are.

A bowden is a setup where the extruder drive motor is mounted in a fixed place on the frame of the printer. The flexible tube connecting the extruder drive motor to the hot-end and nozzle is referred to as a bowden tube. This tube constrains the length between the drive and the nozzle so when the drive pushes on the filament the force is applied to the hotend. This enforces that when length x is pushed through the drive, the same length x is pushed into the melt zone to have a controlled amount of melted filament pushed out of the nozzle. In this configuration, the only way to pull filament off the spool is by the extruder drive motor pulling on filament because it is presumed that you aren't moving the spool around during a print relative to where the frame of the printer is.

On a direct drive setup the extruder drive motor is mounted to the moving hot-end and nozzle. In these configurations the distance between the extruder drive motor and the hotend is fixed, usually around a couple inches in distance, and almost always in a rigid fixed straight path. (I don't know of any that aren't in a straight path, but I don't know every extruder out there including experimental extruders.) In this configuration, not only does the extruder drive motor pulling on the filament off the spool, but the motion of the extruder assembly can pull filament off the spool if it moves away from where the spool is. This can cause situations where the extruder assembly moves closer to the spool, essentially trying to push the filament back onto the spool. But because the spool won't rewind (unless you have a special rewinder type spool holder) this extra slack can allow a loop of filament to fall off of the spool, potentially causing tangles to happen with the loose filament on your spool holder.

The reverse bowden is one of many ways to mitigate the last issue in the previous paragraph. It is a bowden tube that is connected from the frame of the printer to the input of the extruder drive motor. This constrains the distance between the frame of the printer and the drive motor so the motion of the extruder assembly doesn't change the distance from the extruder assembly and the spool. Often if your direct drive printer doesn't have a reverse bowden then someone out there has designed a setup and you can download the bracket parts that fit on your printer from Thingiverse or similar sites to print them out yourself.

I've played with the idea of installing a reverse bowden on my i3 MK3S, but have yet to have a failure related to not having a reverse bowden, but then again I still use the spool holder that came with the printer so the spool is above the print area. This reduces the amount of potential tangle points along the filament feed path.

BTW, if you do go for the MINI, that is a bowden drive setup so there is no need to add a reverse bowden setup to it.

See my (limited) designs on:
Printables - https://www.printables.com/@Sembazuru
Thingiverse - https://www.thingiverse.com/Sembazuru/designs

Another description of reverse bowden - https://e3d-online.com/blogs/news/battling-bowden-tube-physics