Launched in 1954 with primarily doctors as their target clientele, the Ref. 6556 Oyster Perpetual Tru-Beat was one of Rolex's rare attempts at watch complications. The dead-beat complication was developed to give the mechanical watch movement a tick every second instead of a smooth sweep, giving the watch increased precision for more accurate time measurement and pulse reading.
Only featured in the Ref. 6556, the caliber 1040 was developed from the the caliber 1030, featuring two gears attached to the seconds pinion, separated by a spring in between. A small pallet fork locks and unlocks the gears, reducing the usual 5 small sweeps every second to one big spring-loaded jump.
Ironically, the complication and features developed to be the foundation for this model ultimately became its undoing as consumers scorned the reference and its ticking seconds hand for its resemblance to an ostensibly inferior quartz watch.