You must first select the chronograph function on your smartwatch. Press several times on the 3 o’clock pusher to get to the chrono function. 

The chronograph is started and stopped by pressing the pusher. It is reset by pressing the 4 o’clock pusher, when the chrono is stopped.

When the chrono runs, pressing the 4 o’clock pusher will freeze for 10 seconds the intermediate time.
 
Count-up/down counters are started and stopped the same way.