The flyback is a chronograph complication that compresses three distinct operations — stop, reset, and restart — into a single button press. In a conventional chronograph, the sequence is obligatory: you stop with the 2 o'clock pusher, reset with the 4 o'clock pusher, and restart again with the 2 o'clock pusher. With flyback, you press once, and the hand returns to zero while the mechanism has already restarted.
The function was born in the 1930s for military aviation. The pilot had to measure successive route segments in rapid succession, never taking too much attention away from the instrument panel. Three separate gestures were a luxury that couldn't be afforded. The English term describes exactly what you see: the hand flies backward, then starts again.
The heart of the mechanism is the heart cam, mounted on the shaft of each chronograph wheel. When the reset hammer strikes the cam, the asymmetrical shape of the profile guides the wheel along the shortest angular path to 12 o'clock, regardless of where the hand was located. This happens in a fraction of a second, while the escapement train has never stopped. Some manufacturers have replaced the traditional hammer with rack and spring systems — Audemars Piguet did this in the RD#5 — to make the return smoother and reduce hand bounce.
From the bench, I've seen flyback chronographs where the return isn't perfectly at zero: the hand stops with a deviation of one or two seconds. It's almost always a problem with hammer calibration or cam wear, not a design flaw. On vintage movements, this check is the first thing I do before any other evaluation.
Flyback isn't only in pilot's watches. You find it in nautical chronographs — useful at race gate timing — and in motorsport contexts where lap times are measured in tight sequence. Some manufacturers have integrated it into the crown rather than a lateral pusher; the Ferdinand Berthoud FB 2TV even combines it with the balance stop. The constructive solution varies, the functional logic remains identical.
