From someone who has to watch it being used in the mls you are going to be in shock if you think it stops anyone from going forward. It isnt an invisible wall its a starting block just like in track. They may not get within 4 yards of the kicker but there wont be an instant where they are 10 yards away.
As someone who also has to watch the Philadelphia Union in the MLS as well as Hull City when I am in the UK, the only thing that is certain is the neat little circle that the ref draws first to position the ball. Then he marks a line 10 yards away that can vary anything from 7 yards to 15 yards depending upon his idea of what 10 yards looks like.
What if it's a windy day and the spray goes in the players eyes? I can see a point where players in the wall have to wear safety goggles. Health and safety you see.
