In any system the control of fan speeds is based on a TEMPERATURE measured by a sensor. In the CASE ventilation fan situation, the SYS_FAN or CHA_FAN headers normally are guided by a temp sensor on the mobo. The system changes the fan speed according to what temperature is measured. If you add a fan that CAN help deliver more air flow and cooling, the automatic system may simply run all the fans slightly slower to maintain the SAME temperature target they always had. But what that DOES do is, when you increase your workload and get to high cooling requirements, the added air flow capacity CAN keep up with the higher heat load by simply speeding up all the fans.