The flyback (or retour-en-vol in French) function is a complicated mechanism in the movement of chronographs. It is used to reset the stopwatch hand instantly to zero during an ongoing measurement, only by one press of a pusher. After releasing the pusher a new measurement immediately starts.
The usual sequence of "stop", "zero reset" and "start" steps are combined into a single one. (See also: Mono-Rattrapante.)
This handy device was and is particularly favored especially by pilots to measure intermediate times quickly and without much operation effort.
Most flyback movements are chronographs which can be stopped for reading. One unusual application was the Jaeger-LeCoultre Reverso Memory, where Cal. 862 featured a continuously-running 60-minute timer on the reverse side with a flyback button.