In an babel preview buffer opened with 'org-babel-expand-src-block, <C-cC-s> ('org-edit-src-save) writes the expanded view back to the org file, with all the variable expansions, etc.
I may be wrong, but i don't remember this behavior in older versions of Org, and this has bitten me a couple of times, breaking babel code blocks with dynamically set variables (:var …).