Basically Apple have modified the Unix system underneath to allow hard links to directories, something that was (I think) never done before (at least in Unix).
They also added a notification system so that apps can register and get callbacks about various types of filesystem events. Put them together and you can make Time Machine-like stuff.
The article explains it much better than I can, of course.
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@Fraser - here's good article on TM and its internals
Fraser,
Ars Technica did a very detailed review of Leopard (warts and all) and the Time Machine part of it is here:
http://arstechnica.com/reviews/os/mac-os-x-10-5.ars/14
Basically Apple have modified the Unix system underneath to allow hard links to directories, something that was (I think) never done before (at least in Unix).
They also added a notification system so that apps can register and get callbacks about various types of filesystem events. Put them together and you can make Time Machine-like stuff.
The article explains it much better than I can, of course.