VARs, or Video Assistant Referees, watch football games on a screen from inside the stadium, with the benefit of slow-motion replays, and communicate with the match referee to let them know the correct call to make on challenging decisions, such as red cards and penalties.
It is currently being used in Italy's Serie A, the German Bundesliga, America's MLS and various other leagues around the world.
It has now been announced that VARs will be introduced into Ligue 1 in France next season and it is surely just a matter of time before the system is adopted in the English Premier League too.
Proponents say it will lead to more accurate decision-making and lessen the impact of human error (Andre Marriner sending off Keiran Gibbs instead of Oxlade-Chamberlain springs to mind). Referees are naturally going to be influenced by the collective disbelief by home fans every time a decision goes against them. The game moves incredibly fast and referees do not have eyes on the back of their heads. Sometimes, they miss things.
In an era where there is so much money on the line in Premier League football, these refereeing mistakes can be incredibly costly. So why have we not introduced VARs yet?
Well, the purists argue that human error is part of the beautiful game. This is a strange argument that seems to suggest one loves football not despite the injustice it is riddled with, but because of it.
There are the practical difficulties, such as delays in matches whilst VARs watch replays and consider the correct ruling - but these can be minimised and worked-around. The VAR is not always consulted - only when the match official feels he may have missed something - and as technology improves delays will be reduced.
Some argue that not all decisions are clear cut. Any episode of Match of the Day will demonstrate that, despite several hours, numerous slow motion replays from various angles and a team of expert pundits, there are some incidents in which it will never be completely clear what the correct outcome should be. Was the referee right to send Mane off for a high boot earlier this season, when it appeared to be a 50/50 challenge? If so, should Matt Ritchie not have seen identical punishment for the same offence a few weeks later?
That said, the role of the VAR is to provide the match referee with a more accurate picture of what happened. They would show the referee how high Mane's boot was, not how to interpret the rules. The role of the referee is to adjudicate, the role of the VAR is to communicate their observations.
There will still be debate about the application of the rules - that is why lawyers have jobs - but one thing is clear, knowing the facts makes a judge's job a lot easier.