There's an acronym in lexicography that I can't remember right now, but the effect of it is that people try to ascribe a nautical origin to everything. I worry this might be a little of that, but it's a fun story regardless.
Hacking movements gained popularity between WW1 and WW2 due to army interest (and during WW2 US mil-spec for watches required a hacking feature, leading to e.g. the Elgin A-11 hacking watch).
In the heyday of mechanical marine chronometers (late 18th century through early 20th, roughtly), common use was to set a hack watch to the sheltered ship chronometer, then go above deck with the hack watch to make observations.
The link between these two facts is more tenuous, and basically boils down to: watches with a hacking mechanism would obviously be really handy for marine use, given how the hack watch was used; and military terminology routinely bleeds between the services, so transposition from a navy to an army context around WW2 wouldn't be surprising.
As for why the Royal Navy called these watches hack watches... I pretty much just ran down the dictionary definitions of "hack", including the archaic uses that would have been commonplace in the late 18th century, and made an educated guess. I would hope that there's a book out there somewhere about this era of sailing that has a more substantiated take, but I don't have such a reference myself.
This has nothing to do with watches per se but I was wondering which war movies include soldiers shouting "Hack!" because I can't remember ever hearing it in a film.
Here's a memoir from a retired USAF airman where he recounts what the hacking procedure looked like, though: https://code7700.com/2023_time_hack.htm
* each person in the room pulls up the stem when their individual second hand is pointing at 0, and also sets the time to the prearranged hour/minute,
* then everyone waits a bit longer until this is done;
* after another minute, everyone pushes down the stem simultaneously
Is there an alternative where you could have some way to force your second hand to the 0 position rather than waiting up to a minute for it to get there on its own then freeze it?
I do see why hacking is nice for time zone changes.