The purpose of the build out line is to promote player development- allowing a team to play the ball out of the back in a pressure-free (or less pressured zone) at the start of goal kicks and when goal keepers have possession of the ball.
Once a goal kick has been called by the referee or once the goalkeeper takes possession of the ball, the defending team (the team without the ball) should immediately run behind the build out line.
No player of the defending team may cross the build out line until the ball has been put back into play and the ball has left the penalty box area. When the goalkeeper is in possession of the ball, the goalkeeper may put the ball in play at any time and they do not have to wait until defenders are all behind the build out line first.
Finally, at U10, the build out line, and not the halfway or center line, is used to denote offside violations. In other words, an attacker cannot be penalized for offside in the zone between the halfway line and the build out line, only in the zone between the build out line and the end line (goal line.)
There is no punting in U10. This is a rule, and the referee is instructed to call this as a penalty, resulting in an indirect kick for the opposing team.
The no punting rule is intended to encourage teams to use their foot and passing skills to play the ball up from the back of the field, to not simply give possession of the ball to the opposing team, and finally to discourage heading the ball at the U7-U10 ages. Starting in U12, goal keepers may punt the ball, though heading is still not allowed until U14.
AYSO started instituting the use of build-out lines in all 10U matches beginning in Fall 2017. U.S. Soccer has included the build-out line in its mandates to improve the coaching and playing environment for our kids. The build-out line promotes playing the ball out of the back in a less pressured setting and aids in developing possession and decision-making skills.
Section L of the AYSO National Rules and Regulations explains the build out line: