Wiktionary
n. 1 (context chiefly telecommunications English) An change made to a complex system with immediate effect, one made without a phase in period. 2 (context sports English) (rfdef: English)
Wikipedia
A flash cut, also called a flash cutover, is an immediate change in a complex system, with no phase-in period.
In the United States, some telephone area codes were split immediately, rather than being phased in with a permissive dialing period. An example is telephone area code 213, which serves downtown Los Angeles and its immediate environs, split in January 1951 into 213 and 714 all at once. Another example is an immediate switch from an analog television channel to a digital television channel on the same frequency, where the two cannot operate in parallel without interference.
A flash cut can also define a procedure in which multiple components of computer infrastructure are upgraded in multiple ways, all at once, with no phase-in period.
Usage examples of "flash cut".
He swayed a little, leering at me, and for all the reek of booze, the flash cut of his coat, the watch-chain over his flowery silk vest, and the gaudy bloom in his lapel - the marks of the vulgar sport, in fact - the little eyes in his fat cheeks were as hard as coals.
At that moment a blue flash cut down it, ripping the pressure sheet, showering livid yellow sparks.
A flash cut right by the dwarf, frightening the pleasure from him.
A noise and brilliant flash cut off his question and made Ling spin around.
A blue-white flash cut the runner's legs from under him, long before he reached safety.