This is a military term referring to watches that stop the second hand, to allow for more accurate synchronization between two watches. In the most common type of hacking watch, when the crown is pulled out to the time-setting position, a lever is moved which contacts the rim of the balance, thus causing the movement to stop.

Other methods used to stop the movement as well, including brakes on the third or fourth wheel.

Dornblueth & Sohn have developed a complexity of the hack feature called the "Quattro Arret", where the second hand will continue to advance to the next 15 second increment and then stop once the crown is pulled out.

(portions used with permission of TimeZone)