The dark requirement
Misnomer
Short-day is the old label. What matters is a long enough night
Hours
Flowering starts after at least 9-11.5 hours of dark, variety dependent
Streak
That critical night must land 5-7 days in a row
Indoors
The flip is simply giving that dark period on a timer and keeping the night truly dark
Phytochrome in two forms
Pr
Absorbs red near 660 nm
Pfr
Absorbs far-red near 730 nm
Red hit
Pr absorbing red becomes Pfr
Far-red hit
Pfr absorbing far-red becomes Pr
In the dark
Pfr slowly becomes Pr over about two hours
Sundown
Sunlight is richer in 660 nm than 730 nm, so dusk leaves the pool as Pfr
How the flower signal is released
Need Pr
The Pr form is required to release the flowering signal (the book uses florigen as the name)
Overnight job
The long night converts dusk Pfr into Pr and finishes the follow-on reactions
Why 9-11.5 hours
That is the window needed to finish the conversion and the extra steps
Several nights
One long night is not enough; the regimen must repeat for several days
Night interruption and far-red rescue
Red flash
A pulse of about 660-666 nm instantly turns Pr back to Pfr and undoes that night
Far-red undo
A following 730 nm exposure turns the pigment back to Pr so the night can finish
Head start
Intense far-red at lights-off skips the two-hour spontaneous conversion and can cut the dark need by about two hours
Grower takeaway
Light leaks, phone lights, and door cracks during flower night can stall or reverse the flip