When you flush a hydroponic setup, it simply involves draining off the existing nutrient solution completely and replacing it with a fresh nutrient solution which will be of the same composition as the previous one. However, when you are flushing a medium-based setup, then you will need to provide 4-5 times the water that you normally provide it so that all the excess mineral salts that have accumulated on these mediums such as stonewool cubes or coco coir is properly drained off. This would then lead to the plant getting proper access to nutrients that you provide it without the fear of salts blocking their way.