The Umpire Decision Review System (or DRS) is designed so that teams can review decisions that they feel the umpire may have got wrong. However, there is a catch, in that the team that asks for a review loses the review if the umpire decision stands.
In a test match, each team gets 2 reviews per innings for the first 80 overs, and should they exhaust one or both of them, their quota of 2 reviews is replenished. By providing a limited number of reviews, which are lost when unsuccessful, the teams are forced to judiciously use their reviews.
Suppose a player had a way to know every single time if an umpire's decision is correct or not, would that not be unfair to the other team? Unlike the players on the field, the players and support staff in the dressing room have access to TV replays, which often show Hawk Eye predictions for leg-before dismissals. A batsman can be given a signal from the dressing room, where TV replays are easily available, on whether he should review his decision or not and this would make it easier for the batting team to get the review correct each time. This way, the risk associated with the use of reviews is lost and players can always look to the dressing room to review successfully each time.