In watchmaking, synchronization is not a single operation: it's a family of technical solutions that all answer the same practical question, namely how to ensure that the watch on your wrist shows the same time as an external reference, or that two internal oscillators beat in harmony with each other.
The most widespread method is hacking, also called stop-seconds or seconds-hack. When you pull the crown, a metal lever touches the balance wheel — or the seconds wheel, depending on the caliber — and stops it dead. The mechanism is simple: you pull the crown, wait for the seconds hand of a reference to reach zero, then push the crown back. The watch restarts at exactly that second. Without hacking, you must instead try to guess the right moment, with a margin of error that can reach thirty seconds. Almost all modern movements, automatic or manual, feature hacking as standard; its absence on a contemporary caliber is almost always a stylistic or cost choice. In chronometer-certified movements, hacking is considered a de facto requirement, not a regulatory one.
A less common variant is the zero-reset mechanism: instead of stopping the balance wheel, it returns the seconds hand to the twelve o'clock position at the moment the crown is pulled. The practical result is the same — synchronization to the second — but the internal kinematics are different and the gesture has more immediate efficacy for someone unaware of the current seconds position.
On the opposite front, that of automatic synchronization, the most precise reference currently available in a wristwatch is the GPS signal. The Seiko 5X83 caliber mounted on the Astron, to cite a concrete example, updates the time and time zone by querying satellites; the declared precision is ±15 seconds per month even without GPS reception, which already makes it better than any unregulated mechanical movement. Solar cells integrated into the dial power the process without traditional batteries.
There is also synchronization between oscillators, which is a different and much older chapter. Christiaan Huygens observed it in 1665 on two pendulums hung from the same axis: they tended to beat in phase, stabilizing each other. Armin Strom industrialized this phenomenon in a wristwatch in 2016 with the Resonance Clutch Spring patented by Claude Greisler. A physical spring connects the two balance wheels: each oscillates autonomously but the mechanical connection synchronizes them in resonance, increasing the stability of the motion and resistance to minor shocks compared to non-contact resonance systems. I am not aware of other manufacturers who have applied an analogous solution in serial production, but the principle is theoretically replicable.