In an empty Red Bull Arena, a group of teenage soccer players scrimmaged under the watch of a FIFA referee. They defended. They attacked. They scored.
But tactics were not the main focus of Wednesday’s exhibition. Rather, under instructions to create difficult calls for the match’s referee, the players also roamed freely offside, played the ball when it went out of bounds and, in at least one instance, handled it deliberately to prevent a goal.
The game was organized by the International Football Association Board, the organization that administers the rules of soccer, as one of the first live tests of a proposed video replay system for the sport.
In March, the board approved, in principle, the rules for the replay experiments. It also announced that leagues and competitions in six countries had volunteered to take part, including Major League Soccer in the United States, the Bundesliga in Germany and the A-League in Australia, as well as selected competitions in Brazil, Portugal and the Netherlands.
Lukas Brud, the board’s secretary, said on Wednesday that interest in the experiments had grown to 20 competitions from about 15 countries. Although Brud and David Elleray, the former English Premier League referee who now serves as technical director for the board, would not name the new countries, Elleray said that they represented “most of the major competitions” in the world.
The system now being tested employs supplementary officials called video assistant referees to aid the current setup of a match referee and two assistant referees, called linesmen. The premise is that, for a limited set of game situations, an assistant referee in a booth with video equipment can communicate with the referee on the field through a wireless headset to assist in correcting game-changing calls.
Elleray made it clear on Wednesday that replay technology would be limited to dealing with clear errors with goals, penalty decisions, straight red cards and cases of mistaken identity.
That list of incidents, Elleray said, was deliberately short.
“It’s not to make sure that every single decision in a football match is correct,” he said. “The question we will always be asking is not, ‘Was that decision correct?’ It’s, ‘Was it clearly wrong?’”
The sessions this week at Red Bull Arena — there were also closed-door tests on Tuesday — produced the mixed results that the rules officials and former referees looking on had expected. Some decisions in Wednesday’s session with the youth players were raised and resolved in less than 30 seconds, and without the referee, Ismail Elfath, ever leaving the field.
Others took longer, and sometimes required Elfath to run to the sideline to quickly review several angles of a specific incident on a large electronic tablet. In each case, he had a brief discussion with the video referee upstairs through his headset, and then either affirmed or overturned his decision on the field.
However, in every case Elfath retained the final say on the call. Only the match referee can initiate a formal review of an incident under current protocols, Elleray stressed, and only the video referee can recommend one.
“The referee could say, ‘No, I’ve seen everything clearly; we don’t need to check it on the television,’” Elleray said.
There will be no coaches’ challenges in the replay rules — according to the rules board, players and coaches interviewed strongly opposed them for fear they would be used for gamesmanship or to bring about intentional breaks in play — and for now there is also no time limit on how long a review should take.
The officials also warned fans that they should not expect to see video replay in use anytime soon; the board will not experiment with competitive matches until 2017, although Major League Soccer’s vice president for competition, Todd Durbin, said Wednesday that the league was eager to get started. M.L.S. has been running its own replay experiments for several years.
Even in two days of trials this week, Elleray and others said that they could see progress. Delays with review calls had grown shorter, and the communication between the referees and the video referees had improved, they said.
“The body language of the referees was clearer” on Wednesday, Elleray said. “They made it much more clear for everybody when something was being reviewed. So there was a significant improvement we saw, which I think shows the benefit of the education, particularly in the early stages.”
The infrastructure is less of a concern. Brud said that officials from the rules board and from FIFA had already visited with the N.F.L. and the N.B.A., as well as a leader of rugby union, which has used video replay for years, to study their systems and protocols. He said they had come away confident that the technology could be tailored to soccer with the right education and training.
“Nobody benefits from rushing this,” Elleray said, “even though some people think that video assistant refereeing is simply somebody sitting in front of a TV screen and telling the referee what was right and what was wrong.”
Besides, he said, video replay is already available to every fan in the world, in the form of high-definition replays from multiple angles that are often televised within seconds of a disputed decision.
“The one man who needs it,” Elleray said, “is the one man who doesn’t have it.”