In the NFL, video review has become a permanent fixture. The NBA and NHL use it to assist referees. Major League Baseball slowly came around.

And now MLS is turning to technology to improve officiating. In doing so, however, the league will attempt to strike a balance between getting calls right and avoiding long delays in a sport that prides itself on continuous play.

Beginning this weekend, a video assistant referee (VAR) stationed at every match will monitor plays and communicate with the head official at key moments. Only four areas are subject to review: goals, penalty kicks, direct red cards and cases of mistaken identity.

“Video review will not change the basic way the game is played, nor are we aiming for 100 percent accuracy,” said Howard Webb, a former Premier League and World Cup referee who is overseeing the MLS initiative. “Video review will improve the game and give referees the help they need.”

Referees have undergone extensive training on the initiative, and MLS has visited each of the 22 teams to explain to players and coaches how it’s going to work.

Here’s how it’s going work:

>> A video assistant located in a secure location at the stadium will have access to every TV angle provided by the local broadcaster. When he identifies a possible officiating error, he will notify the on-field referee via wireless communication.

>> Because a goal or ruling cannot be overturned after play has resumed, the head referee has the power to momentarily hold up the action during the current or next stoppage in play. In which case, he’ll alert players and fans by raising one hand, like a crossing guard stopping traffic, and bring the other to his earpiece.

>> If the play requires review, the head referee will make a box symbol, like a TV screen. He then will have the option of accepting the video assistant’s recommendation or reviewing the play himself on a sideline monitor.

>> Unlike the NFL, coaches are not allowed to challenge a call. Common fouls, corner kick decisions and yellow cards (that do not involve possible mistaken identity) are not subject to review. If they were, the game would grind to a halt.

The aim, Webb said, is “maximum benefit with minimum interference.”

He added, “It’s only for clear and obvious situations, not the ones that are 50-50. We ask the question, not was the referee right but was the referee clearly wrong?”

MLS has joined Australia’s A-League and South Korea’s K-League among top-flight leagues in implementing video review. Germany’s Bundesliga and Italy’s Serie A will follow suit this season. FIFA, the sport’s international governing body, has used it in the past year at the Club World Cup, Under-20 World Cup and Confederations Cup.

Innovation isn’t new to soccer: In recent years, some leagues and major competitions have introduced goal-line technology to help determine whether the ball has completely crossed the threshold. (It’s similar to the Hawk-Eye system in tennis.) MLS does not use goal-line technology, but the video assistant will help alleviate such controversies.

The International Football Association Board, the guardian for the sacred laws of soccer, will decide in the next 18 months whether to formally incorporate the broader video review into the game worldwide.

“Video assistant refereeing is the future of modern football.” FIFA President Gianni Infantino said.

With IFAB’s guidance, MLS began looking into video review three years ago. The Professional Referee Organization, which administers officiating in the United States and Canada, started testing the system in 2016.

The VAR system was tested offline in 130 MLS preseason and regular season matches, second-division games and youth tournaments. In evaluating the first 90 matches, Webb and his staff found that the video assistant checked an average of nine plays per game and flagged a total of 26 (or 0.36 per game) for formal review.

According to Webb, such reviews delayed the game by an average of 76 seconds. In a sport with a running clock, the referee will add that time onto the usual stoppage time in each half.

“We don’t think it’s going to be intrusive,” said Jeff Agoos, MLS’s vice president for competition.

Because soccer is a low-scoring sport, with the result hinging on a few pivotal decisions, the video assistant will, in theory, clarify calls (or non-calls) that directly impact the outcome. For instance, within technology’s limitations, he will help determine whether a player has embellished contact in drawing a penalty kick and whether a goal scorer was onside or offside.

“There’s going to be a learning curve,” Agoos said. “We’ve done a lot of work to try to prepare for this. Nobody knows exactly what is going to happen. The open question is: Does it improve the game? We think it will.”

The introduction hasn’t gone perfectly, however. At the Confederations Cup in Russia this summer, slow reviews and uncertainty about what plays were subject to review irritated and confused some players, coaches and fans.

Nonetheless, FIFA said, video helped overturn six “game-changing decisions” and confirmed about 30 questionable rulings.

“It’s not a silver bullet,” MLS Commissioner Don Garber said, “but it will correct mistakes.”

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D.C. United vs. Toronto FC

Where: RFK Stadium.

When: Saturday at 7 p.m. ET.

TV: NewsChannel 8.

Records: D.C. 5-14-3, 18 points; Toronto 12-3-7, 43 points.

D.C. probable starters: GK Bill Hamid; Ds Sean Franklin, Steve Birnbaum, Bobby Boswell, Taylor Kemp; MFs Lloyd Sam, Marcelo Sarvas, Jared Jeffrey, Luciano Acosta, Nick DeLeon; F Deshorn Brown.

Toronto probable starters: GK Alex Bono; Ds Raheem Edwards, Drew Moor, Eriq Zavaleta, Chris Mavinga, Justin Morrow; MFs Marco Delgado, Michael Bradley, Victor Vazquez; Fs Sebastian Giovinco, Jozy Altidore.