Zenith EL PRIMERO 21 9004 Mode D'emploi page 12

3 > WINDING THE CHRONOGRAPH
Alongside the double-chain structure of the calibre, and the
remarkable accuracy this ensures, the El Primero 21 movement
boasts two other technical innovations: a new, patented start/
stop/reset mechanism composed of three heart pieces, and an
exclusive starter mechanism which ensures reliable operation
under all circumstances, and allows the chronograph second,
tenths and hundredths displays to be simultaneously reset.
Procedure
0
The crown must be in the position, meaning that it should be
pressed against the case.