Hacking seconds is a feature that stops the seconds hand when you pull the crown out to set the time. “Hacking” just means the seconds hand freezes, letting you set your watch to the exact second instead of getting close.
Why it matters
Without hacking, the seconds hand keeps sweeping while you set the hours and minutes, so you can never nail the exact time. With hacking, you pull the crown, the seconds hand stops dead, you line it up to a reference, and push the crown back in at the top of the minute. That's how you sync a watch precisely — the trick soldiers used to coordinate movements, which is why it's sometimes called a hack watch.
How it works
When you pull the crown to the setting position, a small lever swings in and brushes against the balance wheel, stopping it. Stop the balance and the entire gear train — including the seconds hand — freezes. Push the crown back in and the lever releases, so the movement starts running again instantly.
Which watches have it
Most modern mechanical movements hack, but not all. Plenty of vintage watches and some budget or intentionally simple movements don't — their seconds hand keeps running while you set the time. It's not a sign of quality either way, just a design choice. If setting to the exact second matters to you, check the spec before you buy.