Spring Drive is neither a mechanical movement with added quartz, nor a quartz watch disguised as automatic: it's a third category with its own physical logic. Energy is accumulated in a traditional mainspring and discharged through an ordinary gear train, exactly like in an automatic. The difference lies in the escapement: here it doesn't exist, in the classical sense. Instead of a lever and escape wheel, there's a glide wheel that rotates freely, with no teeth to brake it in increments.
Regulating the speed of this wheel is the system Seiko calls the Tri-synchro Regulator. The name describes the coexistence of three forces: mechanical, electrical, and magnetic. The glide wheel, as it rotates, generates an extremely weak electrical current — on the order of a few nanowatts. That current powers a quartz oscillator that pulses at a stable frequency. The quartz continuously compares the wheel's actual speed with its own reference frequency, and when there's deviation, the system closes an electromagnetic brake that slows the wheel until it's back in sync. No battery needed, no external electrical storage needed: energy is drawn directly from the mainspring in minimal quantities and returned in the form of control.
The visual result is a seconds hand that glides without any ticking, with continuous motion. This isn't an added aesthetic effect: it's the direct consequence of the absence of a mechanical escapement. In chronograph calibers with this technology — like the 9R02 — this property allows the hand to stop instantly when you press the button, without waiting for gear teeth to align.
The stated precision is ±15 seconds per month for standard calibers, ±1 second per day for high-end calibers like the 9RA2, which over 120 hours of power reserve reaches ±0.5 seconds per day. These values far exceed COSC criteria for mechanicals and approach those of thermocompensated quartz watches.
Piaget developed a similar principle in the seventies, based on an electromagnetic brake applied to the escape wheel, but the project remained experimental and never reached serial production. Seiko patented the operational solution and brought it to the European market in 1998. No other manufacturer has yet developed equivalent technology in regular production.