Flushing is to get the excess sodium built up in your soil which will lead to nutrient deficiencies.. if you use a full line of nutrients you should flush every 4 weeks for vegetative stage and every three for flowering stage.. most people dont use a full line so there is no need to flush but for the last week of flowering... you can use a flushing product to start the flush or just water unless your using a full line of nutrients then you'll need to use a product like foxfarm sledgehammer... I for one use the complete line of foxfarm.. I use everything but flower kiss in foxfarm dirty dozen kit on top of foxfarm wholly mackerel and foxfarm kelp me kelp you.. I follow the foxfarm feeding schedule for soil as well as use foxfarm ocean forest soil...