As you can see in the pic of mine you can see it has two second hands, this is to split the timing, either to measure the difference in times between two competitors, or to split a lap time.
You Start timing with the right hand pusher and both second hands move together in tandem.
If you pusher again the hands stop.
To reset to zero you press the winding crown.
To use the split complication you use the left pusher to stop one of the second hands, whilst the other keeps going.