If the small push button is pressed and the green status LED is on, the NA-FC1 operates in “no stop” mode, which means that it will not allow the fan to fall below 300rpm, regardless of the speed setting and the input PWM duty cycle from the motherboard, thus avoiding BIOS fan errors.*
*Please note that many PWM fans have a minimum speed higher than 300rpm and as the function of “no stop” mode is to not let fans go below 300rpm, it is only useful for fans which are generally able to go below this speed. For example, a fan with 500rpm minimum speed will act the same with “no stop” mode on and off.