What does the query parameter “sub_confirmation=1” do on a YouTube link?

Question

I recently came across a link to a YouTube video with

?sub_confirmation=1

appended to the link.

What does this do?

Answer 1

What this does is automatically subscribe the follower of the link to the YouTube channel that posted the video.

It's not totally silent as it will popup a confirmation dialog:

so the user can choose what to do.

It's not a very nice thing to do though.

Source