Hack is a word used for it's clarity and shortness to define a point in time. It's most commonly used in flight, when you are monitoring something and somebody else is wielding the stopwatch.
In a briefing, it's done at the timecheck, the "Hack" is said at a particular point in time - usually exactly a given minute ("the time now is "0832 Zulu, 5,4,3,2,1 Hack"). So that everybody can check that their watches are set correctly and the same.
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