"It's here to stop those headline-making mistakes that you remember down the line some time," PRO Manager of VAR Operations and former FIFA referee Howard Webb said. "It's an additional tool."
The VAR experiment, rolled out by the International Football Association Board, allows a fifth referee with access to video of the game to advise the head referee on four types of reviewable decisions: goals, penalties, direct red cards and mistaken identity. Video assistance is only to be provided "for clear errors in match-changing situations." The head referee makes the final decision.
"Just because we're talking about clear and obvious errors or serious missed incidents, it doesn't mean the referees have made bad calls," Webb said. "It doesn't mean that they're not very good. It doesn't mean that they are repeatedly making poor decisions.
"Quite often, what we've seen is that actually the best use of VAR is not when the referee's made a bad call, it's just when they've not been able to see from their angle what the actual outcome should have been."
In addition to practical field sessions on Wednesday, the PRO training camp included discussions and presentations. The referees had also already gone through remote training.
The VAR system has been tested in MLS preseason matches, offline trials, USL games and youth tournaments. It is slowly being implemented all over the world, with mixed results.
The system made headlines in the Confederations Cup final earlier this month in Russia. In the 61st minute, the VAR alerted referee Milorad Mazic of an incident he hadn't been able to see and many believe warranted a red card. After watching the video replay of Chile's Gonzola Jara elbowing Germany's Timo Werner in the face, Mazic issued Jara a yellow card.
"It wasn't a failing of the VAR system," Webb said. "It was a failing in terms of the judgment of the ref on that day. But the criticism was heavier for him than if he had just missed the elbow because he's had a chance to see it again and still came to a decision that most people thought was incorrect.
"So you can see how this adds extra pressure on us to get things more right than it would have been before. We lose that default defense of, we had one view from one angle and therefore, 'You try it, it's pretty difficult.' "
Before the final day of the Confederations Cup, the VAR had identified six game-changing errors and rectified them.
Webb said he doesn't anticipate the VAR to be much of a disruption in MLS matches. In a survey of the broadcasts on three match days earlier in the season, Webb said, his team found 11 of the 29 games would have seen VAR intervention.
In training this week, the referees averaged between a minute and 50 seconds to two minutes and 10 seconds for each review.
How will Webb feel on Aug. 5, the day the system goes live?
"I'll be a little nervous about the way it's going to go," he said, "because we believe in it, and we've worked hard to make it a success, and we're keen for it to be here to stay."