I run molasses and flush for one week until harvest. I also cut the nutrients by half and increase my runoff two weeks before harvest. There is no proven reason to flush before harvest for flavour, however, fertilizers often build up in the soil to toxic levels so i flush every four weeks anyways. The plants will slow down nutrient uptake for the last few weeks of flowering, causing more nutrients to build up in the soil and increasing the potential for a chemical burn which will effect the flavour. This is why often in side by side comparisons, the flushed plant tastes so much better, and so people assume flushing is the only way to go.
I turn the lights off for two days. The reason for doing this is simply to increase stress mildly which stimulates THC production, and to keep the THC degrading light and heat away from your buds. THC does not circulate within the plant, it is produced in glands which react to various changes in the plants environment.
I plan on writing up the results of my own flushing experiment which covers many aspects of the flushing debate as well as the dark period from an objective point of view. I just wanted big tasty potent buds, and now they're weighed up and cured! I'll write it up tonight after it's been judged by a group of my peers!