They are fast because they have a powerful rare earth magnet to propel voice-coil at one end of the moving head arm assembly around a magnetic fluid sealed bearing tower. 

The feedback embedded by a servo written pattern that stays for the life of the drive. It is between every track and between each sector in the form of inner and outer bits gated by the index pulse.  

The position error signal (PES) depends on the balance of amplitude of these two bits or "di-bits" when on-track or PES=0.  

The entire protocol could fill a chapter and all the technology that makes it fast and reliable, could fill a book after decades of improvements.