The project helps you control two fan’s to cool whatever you need. It works by cooling with the first fan when the sensor detects a user adjustable temperature, and it turns the second fan on, when the temperature exceeds 5 degrees over the user adjusted temperature.
I agree with the author on these being the most efficient way of controlling the temperature and to preserve energy. Also each fan will turn off if temperature drops 5 degrees below the user adjusted temperature. You might ask why 5 degrees ? well to avoid the fans turning on or off too often when temperature varied with only 1 degree plus or minus.
Dual Fan Controller Project: [Link]