Becayse the top of the plant gets bent over under the screen and while the top is growing sides the bottom sides are growing up and it creates and even canopy. You can do the same thing with LST (low stress training)
Yea pretty much the top will grow horizontally until it reaches up a few inches then pull her back down and to the next hole..Even some of the side branches of side branches will make nice colas
It will stop growing vertically a few weeks into flowering to focus on bud production...So while doing a scrog you will have to train the plants by pulling under and repositioning them every couple of days until they stop stretching