In the power management settings, passive cooling means the computer slows the processor down before it increases the fan speed, so that the temperature of the processor remains within the operational limits. Active cooling means the computer increases the fan speed to try and keep the temperature down, but if it starts to exceed the operational limits,it slows the processor down. As far as I know active uses more power, but I am unsure whether this is important to you. So in short passive slows the processor, active uses the fan.
my computer is on active.
hope it helps