I know that you have to do it 10 - 14 days before harvest. Do you just give it water without nutes, or do you have to drain the water somehow?
Not Expert Advice, just opinion I would say wait till soil is dry and starving before Flushing it (without nutes) 3 weeks before harvest to allow the plant have any leftover nutes left in the soil. Finish it all with regular watering (no nutes from here on)