When Germans say an den Stellschrauben drehen (literally to turn the adjusting screws), they mean making adjustments or fine-tuning levers or parameters to influence or optimize a situation or system.
Stellschrauben literally are adjusting screws or set screws used to finely tune mechanical devices. Metaphorically, die Stellschrauben drehen means to carefully adjust variables, factors, or levers that have influence over an outcome — usually in management, business, economics, or personal contexts.
It implies finding the right settings or making precise changes that have a meaningful impact. So, when Germans talk about adjusting the Stellschrauben, they mean fine-tuning critical control points or levers in a process or system to improve or change results.