Curiosity: Auf den Tisch klopfen?
One of the most unusual things that happened when I started my studies in Hohenheim was: at the end of class, all students knocked on the table, as it usually knocks on a door. What was that?
I had to ask the fellow next to me (while I was also knocking on the table with the others). Explained: it is like applauding, thanking the class. Another example of the German education!
Remember that this kind of applause is used in other learning environments, not only in the university environment. In other contexts, it is worth the applause as we know it.
Going to its origins, I managed to find a link (below) with some explanations and curiosities. One possible theory is that a few centuries ago, students who held his feathers in one hand, had only the other hand to show their appreciation and gratitude to the teacher. Es ist wirklich interessant!
Additional information here