it stands for 'Screen of Green' it uses the 'sea of green' concept but when you start to vegg you place a screen over the canopy, forcing the branches to clump up under the screen, thereby exposing the undersides of the stems forcing them to grow horizontally so even more nodes get light. when the underside of the screen is full, you remove it, theoratically increasing your yields, I wish I'd known of it before I started to veg as I'm growing a 'sog' setup under flouros. here is a 'SOG' setup, ok a crude one, but it's working