Shortening the day length from 12/12 to 11/13 effectively speeds the plant up because the plant thinks its flowering past the 21st of September (equinox). Plants naturally see decreased hours as the year progresses, so running them shorter than 12/12 will encourage them to push quicker, though you will suffer minorly in the yield department.
Since equatorial sativas originated near the equator where there is very little seasonal difference in daylight hours, running 11/13 tends to ensure that the plant gets the signal to flower. I have read lots of articles talking about minimum night hours of some strains (mainly indicas) being closer to 14 day/10 night, though its strain dependent. On the other side of things, I have seen some people run the last 3 weeks on hazes at 9 day/15 night to simulate the dead of winter to get them to finish off faster.
“Make the most you can of the Indian hemp seed and sow it everywhere.” – George Washington
"Some of my finest hours have been spent on my back veranda, smoking hemp and observing as far as my eye can see." - Thomas Jefferson
"Two of my favorite things are sitting on my front porch smoking a pipe of sweet hemp, and playing my Hohner harmonica.” – Abraham Lincoln