how does it do it?

i changed my saying at the bottom of my posts and it changed it on old posts as well as from now on. i'm impressed and wonder how it is done.

It's magic. Muggles aren't allowed to find out how it works.

Server-side templating.

Outsourcing.

I found all your posts and wrote a new signature on them all with a pencil. They paid me one pint of beer per ten posts changed. Then they photographed me and put the photo to the right of the forum name.