[hunter2_]

I guess the question might be more along the lines of... Does JavaScript watch each checkbox and POST to some URL when the local user toggles one? Does it poll some URL to learn about changes from the server? Maybe web sockets?

[MarkSweep]

The author has another post that describes how the website worked, including how it changed over the course of its existence. But briefly, yes, web sockets are used to stream updates from the server.

http://eieio.games/essays/scaling-one-million-checkboxes/