The instructor uses a mirror to control the sight picture of the shooter. I couln't find a good picture of a German one and the only wikipedia article is German without a WW2 example ( https://de.wikipedia.org/wiki/Zielkontrollspiegel ).
The following photo might be the impuls for this picture: https://www.bridgemanimages.com/de/noartistknown/shooting-training-in-the-wehrmacht-1940-b-w-photo/photography/asset/7324456