>https://boards.4chan.org/pol/thread/388103576
found this in the comments
Explanation of how this works:
It's making use of 2 channels, 1 with a username of 666 and another with a username of watc.
When you remove h from the url, it can't parse the query string (the part after the ?) anymore, as it's a query string for the watch url. So instead, it tries to open the profile with the name watc.
YouTube channel settings allows you to set a redirect URL for a specific channel, so what this user has done is have the channel of watc redirect to the video uploaded by user 666.
That's it really.
EDIT: it works when you remove the c as well because the channel wath is set up the exact same way.