They are not allowed to intervene for every potential infringement of the rules - only the ones that relate to goals, potential penalties, red cards and cases of mistaken identity in the awarding of yellow and red cards.
Every goal at the World Cup is reviewed to ensure the build-up play is valid.
It took more than two minutes on Thursday for the VAR team to confirm the Japan goal was good.OTHER SPORTS
Other sports have different ways to decide if the ball in in play.
In tennis, the camera-based Hawk-Eye system verifies if any part of the ball was touching the line and therefore inbounds.
In basketball, a player can keep the ball in play as long as no part of the player's body is touching the ground outside the court.