Where is %build's cd to build subdir setup come from?

[Toralf Lund <toralf procaptura com>]

When doing rpm build with a common rpm spec file containing %setup etc, 
the "build" execution will start off with something like:

Executing(%build): /bin/sh -e /var/tmp/rpm-tmp.84274
+ umask 022
+ cd /usr/src/redhat/BUILD
+ cd mypackage-1.0


- where mypackage-1.0 is typically "root" of the Source0: filename, 
although it may also be specified via %setup -n or whatever.

Where exactly does the command "cd mypackage-1.0" come from? I mean, by 
what mechanism does "setup" communicate to "build" that it needs to 
change to this directory before starting the actual build?

I'm experimenting with a advanced (or possibly unnecessarily complex 
;-)) build configuration involving a redefinition of "setup", so I 
really need to know how exactly everything is done by default. Or 
differently put, if I redefine setup, I always end up loosing the "cd" 
command in question....

- Toralf