Aggregate time recording

In a IW371446, what does this mean?

Aggregate time recording: You can add stop times together by pressing the
start/stop button again after the first measurement instead of the reset
button.

I thought that by stopping the chronograph to read the intermediate time, the
counter continued to read the second half. instead when I press the start-
stop button again the counter starts from where it stopped.

There isint't.

Thanks