How does it work?

Behind VAR, there will be the VOR - video operation room - where the assistants will be seated, along with four technical operators.

"There will be four video officials. The main VAR communicates with the central referee and can suggest whether he should come and check footage," said Roberto Rosetti, in charge of VAR for FIFA.

"The VAR assistant No 1 follows the match live, the No 2 assistant deals specifically with offsides and a third assistant is responsible for supporting the main VAR, to verify the respect of protocol and ensure good communication between the team," added the Italian.

FIFA will have a single operational center - as is the case in the Bundesliga - which is in Moscow and connected to all stadiums via a fibre optic network.

Collina - formerly considered one of the world's finest referees - also stressed that VAR officials would never have to cover more than one match a day.

"I was asked why the VARs would have to wear a referees' kit. It's because they sweat like they do on the pitch.

"It's not like watching a game on the couch while drinking coffee," said the 58-year-old.

The question of offside decisions remains extremely complex.

"It all happens so fast, that now we can say: 'If I really have a doubt, I let the player go and if he scores, we'll check the video'. It removes pressure," said French assistant referee Cyril Gringore.

Two additional cameras will be used at the World Cup, exclusively dedicated to offside decisions.

They will be "installed at a height to reinforce an area that, despite the 33 cameras used by broadcasters, was not covered optimally", said Sebastian Runge, head of FIFA's Technology Innovation Group.

In all cases, messages will be broadcast on giant video screens throughout the stadiums so spectators can see what decision is reached by VAR.

No replays will be shown until the on-pitch referee has made his decision.

"No slow motion will be broadcast before the decision is made; we don't want the referee to be influenced by the crowd," said Runge.

Agence France - presse