[VoiceOps] What does an ALG actually do?

I realize that an ALG is a hack in a router that is supposed to allow SIP packets to go through a NAT router. I also realize that for modern SIP equipment, ALG usually causes more problems than it solves, and that it's described in RFCs 2663, 3424, and others. What I can't find anywhere is what a SIP ALG actually does to the packets. Is that written down anywhere, or is it just network folklore?