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."

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."