Pushing a client

How do you push a client?

For example, how does www.meebo.com know when someone sends an IM to you
and send that to your browser. Using firebug, I noticed they do only
have a periodic check of 30 seconds. They do not really have something
polling every 1 second. Do they open a port on your computer when you
log in , so your computer acts as a listening server?

Thanks

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