IWC Schaffhausen Da Vinci Mode D'emploi page 24

Operation of the chronograph
Start: Start the chronograph by pressing the start-stop button.
Fly-back: With the chronograph running, a new measurement can be
started immediately. This requires the reset and fly-back button to be
depressed fully with the chronograph running. All three chronograph
hands are then reset, and a new measurement is started as soon as the
button is released.
Stop: The running chronograph is stopped by depressing the start-stop
button.
Reset: Depress the reset button fully. All the chronograph hands will then
fly back into the zero position.
Aggregate time recording: You can add the stop times by pressing the
start-stop button again after the first measurement, instead of the reset
button.