The build-out line is used to promote playing the ball out of the back in an unpressured setting. When the goal keeper has the ball, either during play or from a goal kick, the opposing team should move behind the build out line. The rule is designed to increase player development through more meaningful player touches in matches, and is also intended to reduce goals from these scenarios.
August 2019 UPDATE
Download the Build Out Line (No Poaching) Rule (WYS All Levels League Implementation 9U,10U,11U &12U).
Important changes for goal kick: as soon as ball is touched and moves, opposition can enter the build out line. The team in possession can place players inside the penalty area during a goal kick. No longer required to have ball go outside of penalty area before the ball is touched a second time.