Atletico Nacional and Kashima Antlers' Club World Cup semi-final will go down in history as the first match to use video replay, as FIFA experiments with a system to reduce referring controversies.
The system is still in its infancy and is not expected to be rolled out before 2018, but this is how it currently operates.
A group of assistants follow the game from a video room, studying each incident in detail, and they are able to call the referee on his earpiece if they spot something odd, while the official can similarly ask those upstairs to look into an incident.
In each case, the referee can either rely on the feedback from the video room or he can pause the match, go to the sideline and check the controversial incident out on a tablet, after which he makes a decision.
Yet not every kind of action is to be reviewed, with only the most important aspects of the game being looked at in order to reduce the number of stoppages and keep up the rhythm of matches, which is the main concern players have had with the new approach.
How is video replay used?
1. The referee asks the video assistants for a second opinion or the assistants recommend that the official reverse a decision.
2. The video assistants replay an incident and describe to the referee what they're seeing on the screen.
3. The referee either makes the decision based on the information fed to him through the earpiece, or he goes to the sideline to take a look himself.

When is video replay used?
Goals: The video assistants will determine whether a goal was valid or not, looking for any fouls in the build-up.
Penalties: The video assistants will make sure that there are no wrong decisions when it comes to the awarding - or not - of a penalty.
Cards: The video assistants will check to see if there has been a mistake regarding the sending off of a player.
Mistaken identity: The video assistants can advise a referee if he has booked or sent off the wrong player.