It's literally like having a browser add-on enable comments on a website by appending the Reddit/HN thread after an article [1]. But with the added benefit of the user being able to choose from a number of algorithmic/community moderation strategies to apply to the existing comments in order to show/hide/rank them.

[1] This add-on actually exists for YouTube/Reddit: https://addons.mozilla.org/en-US/firefox/addon/reddit-on-you....