You're using huge pots. Assuming that your pots are full, you have a lot of soil to flush. The idea is to completely wet the entire area of the pot, so that you get a decent amount of runoff. This lets you know that you've more than likely run clean water through every inch of that soil. You should have around a half gallon or more of runoff. After you do this once, you can water them regularly with clean water. It's to try and break up nutrient pockets that form in soil over time, so they will either flush out, or be used.
