In conventional watches a mainspring powers a balance wheel that spins back and forth, each oscillation clicking an escapement that moves a gear train forward causing the hands of the watch to click out little discrete units of time. Look closely and you'll see the seconds hand moving in jittery little increments. The constant stop-start and reversing motions are a major source of wear and inaccuracy.
Spring Drive technology uses the mainspring to spin a tri-synchro rotor continuously in one dirrection only. As the rotor spins it drives a gear train that moves the watches hands. The spinning rotor is also a micro-generator of electricity, used to power an extraordinarily effiicient clocking chip that regulates the speed of the tri-synchro rotor. This chip, which runs on a mere billionth of a watt, monitors the speed of the rotor and applies a braking force as needed to maintain accuracy.
The effects of all this can be seen in use. When first wound up the tri-synchro rotor begins spinning very rapidly and the seconds hand moves very fast, about two or three times correct speed. Within a few seconds though the chip has powered up and brakes the rotor and the seconds hand slows to reflect the correct passage of time.
The mainspring is wound by an automatic winding system similar but improved over versions used in other automatices. Spring Drive watches have a power reserve of 72 hours, the amount of time they will run without winding, and feature a power reserve indicator that very accurately shows how much longer the watch will run without winding.
While Grand Seiko officially reports accuracy of 1 second per day or about 30 seconds per month, actual user results are typically within only one or two seconds per month!