Petit prélude 926

[landorrano]

Hello.  Along with my daughter, I am working on JS Bach little prelude 926.   Absolutely fascinating, this music.  I would like to better understand what Bach is doing in this piece, which I believe is basically an exercise, as all of the Petit Preludes. 

I imagine that Bach dictated these pieces, his students copied them down.  I don't believe that he said, that sounds good, go work on that.  He invested these pieces with a definite content.  It strikes me that there is a movement or a progression from tonality to tonality, from key to key I think you say in English, that gives a tremendous dynamic to this little piece. But how to understand this progression?