In this context, since Remarque is speaking of automates, the term "Abschnarren der Feder" describes the sound of a mainspring as they were used in toys, watches and similar devices. It is an image for doing something in a very mechanical way thereby loosing tautness and energy. schnarren=to buzz https://en.wikipedia.org/wiki/Mainspring

I never heard that word. Maybe you mean "abschnallen"?