Pulse Width Modulation Fans. 

Pulse Width Modulation (PWM) is a common technique for speed control which can overcome the problem of the poor starting performance of a motor.

PWM for motor speed control works in a very similar way.  Instead of supplying a varying voltage to a motor, it is supplied with a fixed voltage value (such as 12V) which starts it spinning immediately.  The voltage is then removed and the motor 'coasts'.  By continuing this voltage on/off cycle with a varying duty cycle, the motor speed can be controlled.