The rattrapante is a mechanical complication, not a simple chronograph accessory: it requires a second train of hands, a friction system and at least one dedicated pusher, which makes it structurally different from any single-hand chronograph. The name comes from the French rattraper, to catch or engage, and describes exactly what happens: a stationary hand is caught up and realigned by the one that continues running.
Classic operation works like this. When the chronograph starts, the two central seconds hands depart overlapped, indistinguishable. Pressing the rattrapante pusher — usually at 10 o'clock or positioned on the caseband — the upper hand locks while the lower one continues. You read the split time, press again, and the stopped hand jumps forward to reengage with the running one. The operation can be repeated as many times as you like without ever interrupting the total measurement. This is the heart of the complication: the ability to record consecutive intermediate times on a continuous sequence.
From the workbench I've seen rattrapante mechanisms with double column wheels and horizontal friction — the Patek Philippe 5370R is a technical reference on this point — and others with pliers systems, called pliers clutch, where the pliers physically embrace the rattrapante hand's arbor to lock it. These are different solutions with different behaviors under magnification: the pliers tend to leave marks on the arbor over time, horizontal friction is gentler but more complex to regulate.
Omega brought thirty rattrapante chronographs to the 1932 Los Angeles Olympics, and that says a lot about where practical use of this complication was born: sports timing, where measuring split laps without losing total time was a real necessity. Today you still find it in competitive contexts, but also on pure collector pieces like TAG Heuer's Monaco Air 1 and on non-chronographic variants.
Why I say non-chronographic: the rattrapante term has broadened. Parmigiani developed a rattrapante GMT where the two hour hands overlap at rest and separate on command to show a second time zone, then reengage with a crown pusher. The same brand produced a Minute Rattrapante where the hidden hand functions as a countdown on the minutes. The mechanical logic is the same — two overlapping, separable and resynchronizable indicators — but the application is completely different from a chronograph.
One note: some sources confuse rattrapante with flyback. These are distinct complications. Flyback resets and restarts with a single gesture; rattrapante doesn't reset anything, it divides. They should not be used as synonyms.