The English term deadbeat seconds indicates a mechanical complication in which the seconds hand advances in discrete impulses, one per second, stopping sharply on each marker before jumping to the next. It is not a defect, not an imitation of quartz: it is a deliberate design choice, born in eighteenth-century observatory regulators and perfected by the great marine chronometrers, foremost among them John Arnold.
The fundamental distinction to keep in mind is this: in a conventional mechanical movement the seconds hand flows in an almost continuous manner because each oscillation of the balance wheel produces a small advancement; in deadbeat seconds, instead, an intermediate component — a spring, a pinion, a star wheel system — accumulates the energy of multiple oscillations and releases it all at once every exact second. The hand does not tremble, does not oscillate: it stops, waits, jumps. The effect on the bench is immediately recognizable and, for those who know how to look, it is more difficult to construct well than it appears.
The architectures used to achieve this result vary. In Habring²'s A11 caliber an additional spring with dedicated gearing releases the energy every second; in the Garrick S3 a deadbeat spring connects the center wheel to the reverse pinion, visible from the caseback; in the A. Lange & Söhne Richard Lange Jumping Seconds a five-pointed star rotates every five seconds and a lever called a flirt is locked and released by its points; in Rexhepi's RRCC 02 caliber the mechanism is powered by a separate second barrel, serving as a reset function. Jaeger-LeCoultre calls the same function True Second; De Bethune integrates it into the Kind of Two GMT as jumping dead-seconds. The names change, the mechanical logic is the same.
It is worth noting that deadbeat does not concern only the seconds: Jaeger-LeCoultre applied the same principle to the minute hand in the Calibre 184, where the gyrotourbillon governs an advancement minute by minute equally sharp. And in the Arnold & Son Constant Force Tourbillon 11 the jumping display is a direct consequence of the constant force system, which releases energy every minute automatically producing the characteristic ticking motion.
From the reading perspective, deadbeat seconds allow telling time with second precision without ambiguity: the hand is either still or has already jumped, it is not in between. For this reason they were the standard in observatory regulators before the electronic era. Today they are a complication sought by conscious collectors, not because they are rare in absolute terms, but because they require that the manufacturer knows how to control energy in a clean manner without losing power reserve.