I actually think it does work. You have to do some password logging in (to check to see if youtube's just honoring your vote while logged in) and out due to youtube's way of displaying the downvotes, but with this app you can see them regardless if you're logged into yt or not which I later found. So when I logged out, closed browser (cookies deleted) and then opened browser to the same vid - and not logged in....I could see the downvote in-fact had changed. I did this with both downvoting and then clearing the downvote by upvoting and it does seem to register my change in the vote.