how to get at specfile?

Hi All,

I use a src rpm that requires a special switch (define)
to rebuild properly on my OS (CentOS 4.3 - same as Red Hat
Enterprise Linux 4.3, only cheaper):

rpmbuild --define "build_rhel4 1" --rebuild apcupsd-3.12.3-1.src.rpm

I would love to look at the specfile to see what is
actually happening. Not to ask too stupid a question, but
how do I dig the spec file out of a src rpm? Is there
some special switch I can use with tar? Is there a way
to unpack the thing?

Many thanks,
-T