Six months after successfully making its World Cup debut in Russia, the Video Assistant Referee (VAR) system remains a "work in progress" which keeps sparking controversy across Europe's top leagues.
"The VAR is like an airbag: it can help in case of emergency, but only in case of a real emergency," was how former Swiss referee Urs Meier regarded the system which still draws mixed views on how best to use it. In Germany, where the VAR is into its second season, video assistants are being instructed not to improve decisions - only to eradicate bad ones.
"The video assistant should not ask himself, 'did the referee make a good decision?' On the contrary, the question should be 'did he make a bad decision?" explained Jochen Drees, the VAR project leader for the German Football Association (DFB), in Berlin.
In short, all matters of interpretation must be at the sole discretion of the referee on the pitch and the video assistant can only point out what he saw. So if the VAR reports an error, the referee must watch the replay and form his own opinion. No longer can a red card be shown or a goal disallowed based solely on the video assistant's judgment. The complaint of 'double refereeing' should disappear.
"The referee must under no circumstances make a decision, if he has not looked at the incident himself," insists Drees.
Throughout Europe, the impact of the VAR has generally been positive. "The video assistant referee has helped divide by three the number of errors impacting matches" said Pascal Garibian, chief of France's referees. However, the system continues to polarise opinions and there have been several examples of the technology breaking down during matches.
At the weekend in the French league, Monaco coach Thierry Henry was furious when the VAR stopped working, as his side were thumped 5-1 at home to Strasbourg, when he felt they should have been awarded a penalty.
"There was a foul on Rony Lopes in the box when it was 2-1," fumed the 41-year-old. "The fourth official told me: 'I'm really sorry Mr Henry, but VAR isn't working..."
In Spain on Sunday, Luis Suarez scored for Barcelona in the 3-1 win over Leganes, who were convinced the star forward fouled their goalkeeper with a high foot, which the VAR missed. "A blow to the VAR" was the headline in Tuesday's edition of Madrid sports daily AS. While the VAR system is a feature in the French, Spanish, Italian and German leagues, it is yet to be used in England's Premier League and made the headlines for the wrong reason when used in the English League Cup.
Earlier this month, Tottenham Hotspur head coach Mauricio Pochettino was "unhappy" with a decision that favoured his side as they beat Chelsea 1-0 in the semi-final first leg via a penalty awarded by VAR. Harry Kane scored the only goal at Wembley after the referee relied on the video assistant, but it took more than 90 seconds to confirm Kane had been onside before he was fouled.
"I don't like the VAR," Pochettino told Sky Sports. "We get the benefit of it but after watching the World Cup and another league like La Liga I see that nobody is happy from day one that they started to use it."
Back in Germany, senior bosses are developing stricter protocols for communication between the referee and the video room, to hopefully speed up the decision-making process. "For now, everyone still says what he thinks," admitted Drees.