The base movement keeps the time continuously. The chronograph mechanism taps its motion and drives a separate hand that can be started, stopped and reset without disturbing timekeeping.
Coupling is the first design choice. A horizontal clutch connects the driving and chronograph wheels through an intermediate wheel that swings into mesh; a vertical clutch engages friction surfaces along an axis, avoiding the small jump a horizontal clutch can cause at start.
Control is the second. A column wheel — a toothed pillar rotating one step per push — sequences start, stop and reset with a characteristic smooth action; a cam or shuttle system does the same job with fewer parts and a firmer feel.
Reset works by hammers. Pressing reset drops hammers onto heart-shaped cams on the chronograph wheels, which rotate them to a defined zero position instantly.
Added complications extend the same logic: a minute recorder counts elapsed minutes from the chronograph wheel, a flyback restarts without stopping, and a split-seconds adds a second clamped hand so an intermediate time can be read while timing continues.