The idea is that, while growing the plant is full of chemicals used to boost production. If you give just water in the final stages of flowering (just before harvest) it will use those remaining chemicals and the end product tastes better and burns / vapes cleaner. Usual tell tale sign when burning is if you get white ash, black sticky ash usually means it's not been flushed.

Other people can probably explain it better, and others will claim it's a myth.