Growing with fluorescent lights gives one a good understanding of light to plant distance because having them close to the growth is so critical with fluoros. Even so, HID users appreciate the impact of having growth as close as possible to the light. It's how to make best use of the light you have.
In the process of achieving the above, the shape of the growing canopy would match a line in space where light intensity would be equal as the canopy top meets light coming from the source. With fluorescents, it's a perfectly flat plane. With a stationary HID it's a curved shape, the degree of which depends on the area covered. With an HID on a light mover, it's a perfectly flat plane.
Training is a method of growth control that allows one to shape their canopy. Tying, bending, crimping and topping are all conventional indoor training methods. Training branches to grow where one wants in order to get the desired shape takes time. At best, with conventional methods even though branches are where one wants them, when the canopy is in full flower there are void spaces between the buds where other buds could be growing, but aren't.
The Sea of Green method (SOG), where many plants are used in an effort to eliminate the void spaces between buds, was named from the vision of seeing the process in use. The canopy looks like a "Sea of Green". However, the method is very labor intensive due to the high numbers of plants required.
Either way, extra effort is required to maximize the use of canopy space. The plant's natural shape and the shape we want from a canopy under artificial lights are simply not the same. IOW Mother Nature will not cooperate
The extra effort comes in the form of using more plants (SOG), or training fewer plants so each can cover a larger area.
After finding from experience that I didn't like maintaining the numbers of plants, mothers, and clones needed for SOG I opted for using fewer plants. I had to train but still wanted that Sea of Green horizontal profile and no void spaces in my canopy.
Enter the Screen
When a length of poultry netting is stretched over the grow area, it eliminates the need for conventional training methods. Tying, bending, and crimping are replaced by using the netting as anchors to keep shoots in position. The netting can also be perfectly shaped to make best use of the light. The netting is known as the screen, hence the name Screen Of Green or ScrOG for short.
Plants are topped to promote branching, as the plants grow into the screen and their shoot tips start to grow through the holes in the screen, they are pulled back under the screen and guided to the next hole in the screen to continue their horizontal growth, all the time maintaining the profile of the screen to maximize light use. Growth is extremely robust. While now getting the same light intensity as the primary shoot tips, secondary growth seems to blossom, and from the secondary growth comes tertiary growth, etc., all at the top of the canopy, and all receiving maximum light intensity. How many plants are used depends on how much time the grower wants to take to fill the screen to a point where it will be full with buds at harvest. This will largely depend on the growth traits of the variety he uses, but one can fill a canopy with only one plant if desired.
When flowered, only the slow growing buds typical of the
post stretch phase are allowed to grow through the holes in the ScrOG. The resulting harvest profile is indeed a Sea Of Green but with much fewer plants and the increased yields gained from making use of the void spaces found in a conventionally trained non-SOG canopy.
In a nutshell, the ScrOG concept is easy to understand, but putting it into practice often finds a person wanting for details from the moment he places plants into his system to the stretch phase of flowering. Viewing some resources about those details is advised before starting your first ScrOG project.
lol... after 3 weeks of confusion and impatience.. it's alright and growing 
