Negative Flow control System

Do i know more about nigative flow control system? my questions are as follows:-
1.What is Nigative Flow control system?
2.How does it works?
3. Is the pressure/flow is awailable when Excavator is start but no any function is working?
4. Is the pressure/flow awailable when excavator is in working position?
5. How does the propotional solenoid valve is working to control displacement of a hydraulic pump?
6. From where it gets current?
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