Why a mechanical sweep is "smooth"
In a normal mechanical watch, the seconds hand is geared directly to the escape wheel. The escape wheel ticks once per beat (8 times a second on a 4 Hz movement, 6 times a second on a 3 Hz movement). The seconds hand inherits this rate, so it appears to sweep smoothly even though it is actually stepping in 6-to-10 small chunks per second. By contrast, a quartz watch ticks the seconds hand once per second, in a clearly visible jump. That single-jump look has become the visual fingerprint of "quartz".
What deadbeat does
A deadbeat-seconds watch is mechanical, but the seconds hand is mechanically held back from the escape wheel's normal motion by an extra wheel set with a "trip" mechanism. Eight beats accumulate, the lever releases, and the seconds hand jumps exactly 1.0 second forward in a single move. To the eye it looks like quartz; to the ear and wrist, the watch is fully mechanical, with all the usual amplitude, power-reserve, and service requirements. Dead seconds is the same complication under a different name.
Where it came from
The complication is older than wristwatches. Pendulum clocks have used deadbeat escapements since the 1690s, where it was a precision feature, not aesthetic. In marine chronometers from the 18th and 19th centuries, a one-second jump was actively useful: when reading the time at sea, it is easier to capture an exact second from a stationary hand than from a sweeping one. Abraham-Louis Breguet built deadbeat-seconds pocket watches in the early 1800s, and the complication appeared sporadically in observatory chronometers throughout the 19th and 20th centuries.
Modern wristwatch examples
In the modern era, deadbeat seconds is a connoisseur complication, made by a small handful of independents. Habring2 in Austria offers the Erwin Jumping Second from around CHF 5,500, the most affordable serious deadbeat on the market. Grönefeld's 1941 Remontoire uses a one-second remontoir constant-force device that doubles as a deadbeat. F.P. Journe's Tourbillon Souverain has dead-beat seconds combined with a tourbillon. Andreas Strehler, Arnold & Son, and Jaeger-LeCoultre have all made versions; Jaeger's Geophysic True Second (2014) was the only "industrial" deadbeat on the modern market and is now discontinued.
Why it is rare
Three reasons. First, the complication adds parts and friction without improving timekeeping; it is purely aesthetic / functional in a sea-navigation sense. Second, the small consumer market: most buyers actively prefer the smooth sweep as the visible difference from quartz. Third, it is genuinely difficult to make a deadbeat-seconds movement that is also accurate, because the trip mechanism interrupts the normal energy flow and, if poorly designed, can pull amplitude. A well-executed deadbeat is a watchmaker's flex.
Should you want one?
A deadbeat-seconds watch is for a specific kind of buyer: someone who already owns the obvious complications (chronograph, GMT, perpetual) and wants something conceptually interesting rather than visually loud. The pleasure is in the slight mismatch between what the eye sees (quartz) and what the wrist feels (mechanical). If you already enjoy mechanical sweep, you probably do not want deadbeat seconds. If you find sweeping seconds slightly chaotic, you might love it. See also: Spring Drive, which goes the opposite direction with mechanically-driven, electronically-regulated perfectly smooth sweep.