How it works
Spring Drive replaces the standard Swiss lever escapement (balance + lever + escape wheel) with a 'tri-synchro regulator': a glide wheel, a tiny generator that produces electricity from the gear train's motion, a quartz oscillator that runs off that generator's output, and an electromagnetic brake that controls the glide-wheel speed. The mainspring drives everything mechanically; the quartz crystal acts only as a reference timing source for the brake.
What it feels like
The seconds hand glides smoothly instead of ticking. There's no escapement 'tick'; the watch is silent. Power-reserve operation is normal mechanical (wind it, it runs; let it run down, it stops). It can be hand-wound or rotor-wound. From a wearer perspective, it behaves like a mechanical watch with a smoother seconds hand and significantly better accuracy.
Where to find it
Spring Drive is exclusive to Seiko and Grand Seiko. The 9R65 / 9R66 / 9R86 calibre family powers most Grand Seiko Spring Drive references, including the iconic Snowflake SBGA211. The Credor brand (Seiko's high-end division) has produced Spring Drive minute repeaters and tourbillons. No other manufacturer has Spring Drive technology; Seiko's patents have prevented broad licensing.
Why it matters
Spring Drive is the most accurate mechanically-powered watch ever made. ±0.5 sec/day is roughly 30x better than COSC chronometer spec. The smooth seconds-sweep is a visible engineering signature unique to Spring Drive. The trade-off: it costs roughly 2x equivalent mechanical Seiko Grand prices (Spring Drive Snowflake at CHF 6,800 vs mechanical Grand Seiko at CHF 4,800). For most wearers the accuracy gain doesn't justify the premium; for collectors who appreciate the engineering it's an obvious choice.