Stop-seconds, commonly called hacking, is a mechanism that brings the balance wheel to an immediate halt when the crown is pulled to the time-setting position. With the balance stopped, the seconds hand freezes in place, allowing the wearer to synchronise the watch precisely to a time signal before releasing the crown and restarting the movement. When the crown is pushed back in, the balance resumes its oscillation and the seconds hand continues from exactly the position it stopped.

The mechanism typically works via a small lever or spring that, when the crown is pulled, presses against the balance wheel's rim or the hairspring stud, creating enough friction to arrest the oscillation. The approach varies between manufacturers — some apply pressure to the balance wheel directly, others to the regulator — but the functional result is the same.

Hacking is now considered standard on almost all dress and sport watches, but it was absent from many movements until the mid-20th century. Rolex introduced a hacking seconds mechanism to its Submariner in 1977 with the ref. 1680. Military watch specifications from the 1940s and 50s often required hacking as a functional necessity, since soldiers needed to synchronise watches to the second before operations. Watches lacking the feature are sometimes described as 'non-hacking', which is a mark of either vintage character or budget movement specification, depending on context.