MLS and the Professional Referee Organization (PRO) announced on Friday that live, in-game testing of the Video Assistant Referee (VAR) initiative has begun in select MLS preseason games.

The Video Assistant Referee initiative, as outlined by the International Football Association Board (IFAB), consists of an additional referee positioned in a booth with technology that allows access to video at a finger’s touch from every available broadcast feed camera angle.

A communications system will allow the VAR to alert and advise the head referee of clear errors in four types of “game-changing” situations: Goals, penalty calls, direct red-card incidents and cases of mistaken identity.

The VAR system made its in-game testing debut in MLS on Friday in a 1-1 draw between Real Salt Lake and the San Jose Earthquakes in a preseason exhibition in Tucson, Arizona. The system was called into action on one play, confirming a penalty-kick call against San Jose in the 88th minute (watch above). RSL couldn’t convert from the spot, with forward Chad Barrett sending his effort over the bar.

Fifteen of the 22 MLS clubs will gain exposure to VARs during the preseason. VARs will be in use at the Desert Diamond Cup in Tucson, the Portland Invitational Tournament, the Carolina Challenge Cup in Charleston, South Carolina and the IMG Invitational in Bradenton, Florida. View the full list of preseason matches that will feature live, in-game VAR testing.

In a statement, the league said that the “continued testing will focus on ensuring that communication between the referee and the Video Assistant Referee is clear, precise and efficient, with the aim of reducing avoidable delays to create minimum interference with maximum benefit.”

Following up on the preseason tests, MLS will run offline testing at all 22 venues from the start of the regular season through the 2017 MLS All-Star Game, presented by Target on Aug. 2. Offline testing will not have any impact on the game as VARs will not be communicating with head referees. The league hopes to incorporate VARs fully into MLS matches following the All-Star Game.

MLS Commissioner Don Garber announced at his State of the League Address in December that the league would test the VAR system in regular-season matches prior to the All-Star Game and that they hoped to roll it out fully for the second half of the regular season.

“These additional tests during preseason, as well as the offline tests during the beginning of the MLS regular season, will enable us to refine the experiment protocols and procedures prior to implementation in league matches,” MLS executive vice president of competition and player Relations Todd Durbin said in a statement released by the league.

“During this preseason phase of testing, PRO will be keen on identifying consistent decision-making and fine-tuning referee-to-VAR communications,” added PRO GM Peter Walton. “These valuable trainings will provide real-time review opportunities and will serve as a stepping stone for in-game testing during the MLS regular season.”

The IFAB hopes to make a decision about the VAR experiment in 2018, or in 2019 at the latest.

Further information about the VAR project is available on the FIFA website, along with an FAQ document.