The subtitle of this choral prelude in manuscript Mus Ms 40037 (owned by the Staatsbibliothek Berlin) "Fuga cum contrasubjecto al inverso". Zachow writes a clever fugue, based on the first phrase of the chorale melody as theme. The countersubject, already sounding in the first bar together with the fugue theme, is written in such a way that Zachow can use it in it's inversed or 'upside-down' form as well. In bar 6 the inverted countersubject sounds in the soprano voice, so it's easy to hear. And in bar 24 both forms of the countersubject sound at the same time.
As the fugue theme enters 9 times in this piece and the countersubject 16 times (both forms counted) one could actually wonder wether the counter subject is actually the main theme of this fugue.