Never heard of it. Wouldnt do it personally, might shock the plant. I could be wrong though, I'd have to see it tried.
Are you sure your not thinking of 36 hours of darkness before cut? I've heard of that... but not inbetween veg and flower...
I'm surprised at some of these answers. This definitely is done by some growers and has been discussed at length before here at GC. There are three basic approaches to how to switch lighting from veg to flower. They are in some ways opposites of each other and yet each has a solid logic behind it: 1) Ease into it gradually, such as adding one hour of darkness a day until 12/12 is reached. The theory here is that it mimics Mother Nature. Except Mother Nature goes from summer solstice to 12/12 over about 3 months not one week. Those who use this method agree that it isn't truly mimicing Mother Nature but at least it is somewhat gradual compared to just switching cold. 2) Start the 12/12 off with an extended period of dark such as 36 straight hours. Proponents say that this "kick-starts" the hormonal reaction that induces flowering. Some claim that flowering starts several days earlier after doing this. 3) Just switch from veg lighting to 12/12. It's most convenient for the grower, and proponents say it makes no difference to the plant. I have tried all three and have to say that I didn't notice much difference either way. A 36-hour dark period to start 12/12 will not shock the plant. I have settled into about 31 hours of dark in between veg and flower cycles but only for convenience -- I time my light cycles so that lights are off during the hottest part of the day, so for veg I'm usually running about 6pm until 12pm. For flower I'd like my 12/12 to run from about 7pm until 7am, so on the last "day" of veg lighting it goes off at 12 noon and I don't go lights on again to start 12/12 until 7pm or so the next night. It works just fine.
didnt realize there were so many theories on switchin from veg to flower. i know we've tried 2 of the 3, never noticed a difference from one to the other. coarse one time when we fliped we let temps get way too hot, stressed the poor gals into thinkin was still summer & they kept vegging, but that's a tad off topic... oops