How does a web app like Wunderlist show me what another person is typing as they're typing it? I'm assuming it has some kind of js MVC framework on the client side and a REST API server-side, but are each of my colleague's keystrokes being sent to the server as they type? Wouldn't that be intensive on the server? Or is it some other kind of magic I don't know about?
I think that you are right about the way it could work, but i don't see why it would be so intensive on the server. It's nothing more than intercepting keystrokes of arrows for an online game and moving a character accordingly... And servers support much more than this kind of thing. The amount of information sent is minimal.