A chronograph watch possesses a needle placed in the center of the movement called "sweep second hand", being able to be at will started up, stopped and returned to its starting point.
The chronograph possesses 3 functions:
  

1) Starting up

2) Stop

3) Reset to zero

 

How to Use a chronograph:

1) Start:

Exercise a pressure on the button A. 

2) Stop:

Exercise again a pressure on the button A to stop the sweep second hand in the precise point where it was during the pressure. 

3) Reset to zero:

Exercise a pressure on the button B to return the sweep second hand to its point of departure 

Button A: starting up and Stop (Ruling) 

Button B: put back (handed) to zero of the chronograph