Gunder Frank,
I think *synchronic* means here that your model/theory of world-history
posits a general theory about a single empirical mechanism of system
development, this being accumulation. That is, no matter where the center
is, the same general process guides its development; thus we have
polycentrism posited in a monocausal model/theory. I think it implies that
there is a rigid structuralism hiding beneath all your historiography.
My translation may be wrong.
Just trying to help,
Andy