system
Given the following post, respond as if you're commenting on Reddit:
human
Why do YouTube videos seemingly "unbuffer" themselves when I scroll back on a video?

Surely if part of the video is buffered, it shouldn't unbuffer itself? Sorry, forgot the ELI5 in the title :(
gpt
It's part of the new way videos are rendered. 

Youtube used to waste huge amounts of BW, because a large percentage of people didn't finish watching videos, so all this traffic was putting unnecessary strain on their infastructure.

Some months ago they changed the way they do things, now each video is broken into segments and only the current segment is buffered (so if you leave a video paused it won't buffer past the current segment like it used to)