It is said the 36hr dark period will jump start flowering by a week. When you turn the lights back on to the new 12/12 cycle your plants will show sex within a day or two. I don't do this personaly but have talked with quite a few growers who do.They swear by it. I think it's kinda risky IMOP. As for flushing nutes between cycles.....No I wouldn't do that at all. Keep her on her veg ferts the first week of flower before switching over to bloom ferts. She won't be in flower right off and the little extra nitrogen will help her out until you switch the ferts to bloom.