An additional second hand on a chronograph that can be used for timing in competition. To record the duration of a competition, just stop the fly-back hand. This fly-back hand can be stopped independently and made to catch up ("fly-back") with the other constantly moving second hand. Use repeatedly when needed.
In a chronograph with analogue display, an additional centre second hand which can remain superposed on the other one as it moves, can be stopped independently and then made to "fly back" so as to catch up with the other hand, can be stopped and reset to zero together with the other hand. In chronographs with numerical display, a "function" having the same effect.
An additional hand on a chronograph which moves with the second hand but, can be stopped independently to measure an interval which can then "fly back" to catch up with the other hand.
Usually, a hand indicating a date or time against a scale and then "flies back" to catch up with to another date or time. For example, a hand that "flies back" to the beginning of the month after reaching the 28th, 29th, 30th, or 31st day of the month.
In a retrograde indication, the hand that reaches the end of a scale and automatically flies back again to the beginning. In chronographs, the hands that can be made to return to zero.