It grows the entire time
Some plants stretch more than others during the flower period. Some heavy indicas Just seem to stay the same height but fill out with flowers and sativas seem to go everywhere. But they all are growing and the flowers are coming from the leaf nodes and growing out. So I guess you could say they never grow down the stem.
When you flip the plant to the flower cycle it triggers the plant to start to producing flowers, this starts on day one of the 12 hour cycle. As far as the question of the 12" plant it will continue to grow up and out as it develops its flowers.
Each strain is different and within these strains are pheno's that grow different, so keep growing.
Any one else want to chime in?