So there is a long ptfe tube that runs from the MMU2S down to the extruder. Every time the mmu wants to change filament it must pull the filament out of this ptfe tube and feed it back towards the spool. But if it unwound the spool we would see that in our prints as the spool hit points of load, or worse the filament might unspool and come off the spool. So this buffer gives the rewound filament a place to go without unraveling the spool. I re made mine in clear so i can see the filament inside doing its thing. This short video shows both the filament backing up into the buffer and also it shows my intended setup. I mean to hang all 5 spools on the wall just like this: https://www.youtube.com/watch?v=kDeXqmv8rII