More than making flowering faster (a few days at most) is so they all start at the same time (so you dont have plants with different harvest times occupying your indoors preventing you to start the next badge). The 1 to 3 days of dark before harvest is suposed to boost terpenes since the plant uses the dark cycle to restore terpenes that evaporate during the day. I do that aswell, does it work? Im not sure but I could swear they are smellier after 1-2 days of dark before harvest. Meantime, the beforementioned friend says he notices nothing different with that technique, so I dont know for sure either. Going back to the 1 day of dark before the change of cycle, the plants will do what they do at dark as long as they are at dark, they recognize that light should have appeared but since they cant do day activities they keep doing night activities. In the case of realizing its time to swap, that night activity is basically triggering flower with chemicals. You can easily tell that they keep doing night activities if you for example forget to put the timer in auto and they dont have light, if you open the tent even if they were suposed to be doing day activities they will be "sleeping" with the caracteristic slightly droopy leaves. Also I never said it works

, I said my friend uses that technique and I used it in my current grow and only one is slightly behind and they didnt get stunted or anything, not overly stressed, eating as usual too. That said, it could have been pure luck, with just 1 try I cant tell you it works, he swears by it and I trust him enough to try it, his weed is really, really good. Perhaps its just in his mind, but I figured its worth the shot.