Can anyone briefly explain to me how a Blizzard authenticator works? I'd like to implement something like that for fun, but apparently my google-fu is weak. A link would also be good.

[If anyone cares, I bought some model rocket motors and a TI Launchpad, I have a modem and a landline... Dialing in a launch and requiring the use of an autheticator to do so sounds like great fun. My own pretend nukes.]