IWC Schaffhausen Portugieser Chronograph Rattrapante Mode D'emploi page 23

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Aggregate time recording: You can add the stop times together
by pressing the start/stop push-button again after the first meas-
urement instead of the reset push-button.
Suggestion: In order to avoid placing unnecessary strain on the
movement, the split-seconds hand should not be left in a stopped
position for long periods. It is essentially only used for the meas-
urement of intermediate times or for recording a second finish time,
and should be resynchronized with the chronograph seconds hand
once again after doing so.
P O R T U G I E S E R C H R O N O G R A P H R A T T R A P A N T E