When the lights are switched to 12/12 the plant will receive and process signals to begin flowering.
During the flowering cycle the plant will double or maybe triple in height.
The first 1/3 of the flower cycle is spent in a lush growth spurt, after which the real work begins: producing some bud.
New growers usually miss out on the full potential of their plants by switching to a bloom formula too early. I suggest using a vegetative fertalizer through the "stretch" phase, then switching to a bloom/fruit formula.
In plain english, I'd continue to use my vegging nutes for the 1st 7 to 10 days of flowering, than switch to flowering nutes.
Peace