If the question is what flips a photoperiod plant, it's not 12/12, a plant is triggered to flower long before 12/12.
The fall equinox(12/12) was September 22nd this year, outdoors plants start flowering early to mid-August(14/10) outdoors. I've been growing since the mid-nineties and I have never had a plant wait until the equinox to start flowering, this is normal, not talking about not "fast" strains or anything crossed to an auto either. Cannabis evolved to start flowering before 12/12, late September is way to late to start flowering in most latitudes for seeds to mature before nasty weather or frost kills the plant.