A watch that “hacks” or that has “hacking seconds” is one in which the seconds hand stops when the crown is pulled out. Hacking is achieved when pulling the crown out to the setting position causing a lever to come into contact with the balance wheel causing it to stop and to be held in position. Hacking allows the watch to be set to a reference signal, or synchronized with a second timepiece. Pushing the crown in releases the lever, allowing the balance wheel to move freely.
Hack on a Rolex 3035
The hack on a Rolex is a simple thin piece of brass that is attached on the dial side to the detent lever. When the stem is pulled into the setting position, the hack slides forward and lightly touches the underside of the balance wheel stopping the watch. Simple!
Hamilton 4992b pocket watch with hack
The Hamilton 4992b military pocket watch is designed with a hack. It was used in WW 2 for aviation navigation. The hack was essential in timing for aviation and bombing runs.
ETA movement mechanical hack
Most ETA mechanical movements come with a hack. It is a simple brass lever that is attached to the clutch wheel. It slides forward to lightly tough the balance wheel when the watch is in the setting position.