The hack seconds is not a convenience: it's a measuring function. When you pull out the crown, a mechanical device — in the vast majority of cases a lever or brake acting directly on the balance wheel — immobilizes it. The seconds hand stops. At that point you can compare your watch with a reference signal and restart the movement at the exact moment by pushing the crown. Real gain: you regulate to the second, not to the tenth of a second.
The principle is over a century old, but implementation varies greatly. In mass-production calibers — Sellita SW 200, Oris 561, ETA 2824 — the brake acts on the balance wheel through a spring lever that engages on the second click of the crown. In Glashütte movements, like the Nomos DUW 3001 or certain Glashütte Original Senator calibers, the construction is more refined: some integrate a minute detent, that is, a minutes stop synchronized with the seconds stop, so the minute hand jumps to the next index the moment the balance wheel stops, eliminating ambiguity about the half-hour. It's a solution I've seen on few truly successful pieces; in most cases the minute hand isn't involved and it's up to your eye to estimate the position.
In tourbillons, the hack seconds becomes more complex because stopping the balance wheel means stopping the entire rotating cage. The Carl F. Bucherer CFB T.3000 is a documented example: you can lock the cage to synchronize without having to wait for a complete revolution. A rare function, truly useful only if you want to regulate a tourbillon piece used daily to the second.
Moritz Grossmann patented a different solution for the crown: pulling it out automatically activates the hack seconds, but the crown itself snaps into a locked position to prevent dust entry and dial wear. A separate pusher — at 4 o'clock on the models I know — restarts the movement and returns the crown to winding mode. It's a more articulated system than normal, but it solves a real problem: every time you leave the crown pulled out for a few seconds you expose the movement.
In military and diving contexts — Panerai above all, with calibers like the P.980 — hack seconds was an operational requirement before it even became a commercial standard. Divers and demolition experts needed to synchronize to the exact second before descending. That logic hasn't changed: the function still serves those working with tight timings today. From the bench I've seen bracelets replaced on watches without hack seconds and remounted on pieces with it; the difference in daily regulation is concrete, not theoretical.