Uninterrupted darkness is the key... this is what causes the hormonal response in plant which causes the flip to flower production. Light is irrelevant as long asyou provide enough and don't interrupte the dark period.
it isn't even "12" hours... closer to 10 hours of uninterrupted darkness is necessary. It's just safer to use a 12/12 to avoid any genetic variance. 13on/11off is also safe if you can make use of another 8% DLI. Gassing an already gassed plant won't help though.