The red needle is moved or dragged by the black vacuum needle. Once moved due to vacuum it simply stays where the highest vacuum was generated until you manually re-set it. Because this fuel pump is only running a polishing loop from the tank, through the filter then back into the tank, I can see very easily see when the filter begins to plug. A nice feature of a drag needle gauge is that I do not need the system running to view how plugged the filter is becoming.