A 6 hr dark period may not be enough to hold a plant in flower, usually it needs 8 hrs or so to stay in flower. With alot of strains tho they won't trigger flowering at 8 hrs, it may be necessary to start at 12 hrs of dark to get them to flower but you can then cut them back to 8 hrs or so after that.

While the plant does not have a clock per se, it takes a certian amount of time for certain hormones to increase or decrease, depending on light or dark duration. The plant's hormones act like switches, turning on or off certian processes such as when to go into flower mode. This is determined by the plant during the dark period.

You'll come across some recomendations of put your vegging plants into 24-36 hrs of darkeness before going to flower, this is to insure flowering and supposedly helps put the plant to flower quicker.

The photoperiod controls the rate of growth as you have seen yourself. In simple terms the more light, either intensity or duration will increase growth rates.

But everyone needs to keep in mind these are generalizations; with different strains and individual plants there will often be exceptions to general rules.

One big exception is landrace equitorial sativa's, they can veg very happily at 12/12, just because that is close to the photoperiod they are used to.

Photoperiod is one of the easiest ways to manipulate a plant and it's growth for our benefit. We are just scratching the the surface in understanding flowering btw.