I have 2 edits very close (and therefore part of the same) that I would like to commit separately. Is it possible to break a piece due to magic?
You can correctly set the mark with Ctrl + Space (C-SPC), and magit will only fix the selected part.
You can also use - and + to zoom out or out of pieces in diff (and 0 resets), but the most important information on how to find out is the dominikh answer.