What is a Rattrapante?

A "Rattrapante" is a chronograph with an added second hand, to allow, for example, lap times in a multi-lap event to be read off without stopping the chronograph. A chronograph is usually started with one pusher, which starts both second hands moving (one superimposed over the other). When the operator desires to read an intermediate time, he/she pushes a second pusher. On the dial, one of the second hands stops (the "split" hand), while the main second hand continues. If the second pusher is pressed again, the split seconds hand "catches up" with the main second hand, and is ready to be used again.
Interestingly, three languages describe this function in different ways: "Rattrapante" is the French term for "catch up" (describing the motion of the split hand); the German term for this function is "Doppelchronograph", or double chronograph; the English term is "Split Seconds" (describing the appearance of the second hand when the function is activated).
An example of this function is on the IWC Doppelchronograph or Breitling Chronoracer (a mecaquartz (!) rattrapante).