Pro tip, I think they can tell if you just show up and down vote without watching at least a portion of the video, so just let it play for 30 seconds or a minute or whatever

I just open them in a new window, hit play with the volume down, minimize it.

Come back in a minute or two, down vote then leave.

Sure it is a little more trouble, but when you really want your vote to count for once, its worth it.