In watchmaking, a function is any performance of the movement that goes beyond simply indicating hours and minutes. The distinction that really matters, and which is often overlooked, is the one between function and complication: not everything an watch does extra is a true mechanical complication. Hacking seconds — the stop of the seconds hand when the crown is pulled out — is a regulation function, not a complication; yet on certain vintage calibres its absence makes precise synchronisation to the second impossible.
Functions are divided by purpose. Regulation functions include hacking seconds and quickset date: the latter, which appeared on Rolex Datejust models from certain references in the Seventies onwards, allows you to advance the date without turning the movement for hours. It is operational comfort, but on the bench it makes a difference when you restart a watch that has been stopped for weeks. Measurement functions include the chronograph in its variants — single-pusher, two-pusher, flyback — and auxiliary scales on the dial: tachymetric, pulsometer, telemeter. The pulsometric scale, which allows you to count heartbeats without additional instruments, was originally created for doctors; the tachymetric scale measures average speed over known distances and only makes sense if you know the calculation basis on which it is calibrated.
The flyback deserves a separate note. Technically it allows you to reset and restart the chronograph in a single gesture, without the intermediate stopping step. It originated in military environments — the French Type 20 from the Fifties is the historical example — where measuring consecutive events without losing fractions of a second was operationally critical. On modern calibres like Hublot's HUB1280 or the Moser movement in the Streamliner, the underlying mechanics are the same: an immediate return system that discharges and releases the chronograph train in a single impulse.
Temporal orientation functions include the GMT, which adds a supplementary hand to read a second time zone, and the day/night indicator, which resolves at a glance the AM/PM ambiguity when operating across multiple time zones. There are then less classifiable functions: the secret date on the Girard-Perregaux Casquette 2.0, which flashes a programmed personal date, or the chronograph status indicator patented on certain Type 10s, which visually signals whether the mechanism is running. On the Leica ZM, the crown is pushed instead of being pulled to reset the seconds — a design choice that redefines the gesture of the function itself. Knowing what a function does is one thing; understanding why it was designed that way is another.