How does `undo` know that `keyboard-quit`has been called?

[Raffaele Ricciardi]

I would like to write a command that behaves like `undo`, that is: a 
command that reverses its behaviour when the user calls `keyboard-quit`. 
 I have read the Lisp code for both `undo` and `keyboard-quit` and I 
can't understand how `undo` achieves such behaviour. (I know that I 
could always advise `keyboard-quit`.)

Thank you.