my mate does that he gives them 48hrs darkness b4 harvest he says the plants heal themselfs in the dark,i dont do it myself but he get pukka results
lol well, he's wrong, haha darkness enduces the flowering hormone, which is why the lights go to 12/12, i never give my plants an extended dark period before i harvest, i always do it for 2 days before flushing, the plant will produce more buds, but they will not be mature yet, and would be pointless to smoke, but after another week of 12/12 they should mature enough to be worth smoking, also the best part about giving it darkness before the flush, is that it will still have nutrients to make the buds, and after the flush, when the plants are breaking down the nutrients in the leaves, they will have to use up more of the remaining nutes to finish the new buds, therefore increasing the 'fade' which improves taste...

I find it helps to finish your buds off, and even them out a little,