Amazing video, can someone help me?

this is a video from the guys from "Green house seed company" they are really professionals

he talk about flushing, and then drying for 5 days, what does he mean ? can someone explain please? by the way i believe this is hydro grow

http://www.growhd.tv/video/Kalashnikova-Grow-Video-2011-HD/6c7a695f76989f855fb154594b289033

they are just talking about flushing with just water then letting it dry out for five days. That's no water or feed for 5 days. five days in a hydro set with no water it has to be in soil.Real pro's everything I have been reading about green house it that people aren't happy with the genetics greenhouse has been send out as of late "garbage".