It's more like the plant requires more phosphorus as it enters into flowering and through to mid flower where the amount needed is beginning to taper off. (So no, it's not a trigger or lever that you can pull to induce flowering.) Potassium steps in mid flower to the finish. How does that translate to you as the grower? It all depends. If you're an organic grower, you want that in the soil about a week before you anticipate flipping to flower. If you use salts, there's no need to feed the soil in advance.
The sun, whether it's artificial or real, and the darkness cycle is what triggers flowering. The rest are the "vitamins and minerals" that support the plants growth.