It's called the Sea of Green (SOG) method and is used for rapid harvests with high plant density. When you flip the plants into 12/12 (usually about 7-14 days old) they shoot off in a huge amount of growth. Internode spacing is pretty stretched, but your plant grows at an insanely fast rate and then flower when ready. If you want a quick harvest and you're willing to sacrifice some yield, then it's probably a good way to go.

This method can also be combined with a Screen of Green grow, allowing you to fill up your screens quickly due to the rapid growth of your plants.