Just having someone who is following the game with a chess engine and who has a way to get a single message to you telling you that your opponent's last move was a serious blunder would be enough to give you a noticeable advantage.
For example look at the position in this video [1] from a recent game on Chess.com between Hikaru Nakamura and Fabiano Caruana (the title of the video says Magnus vs Hikaru because the video covers 3 of Hikaru's games in the tournament).
I linked to a spot in the video a little before the part where one simple message could changed the game because the host is explaining what Hikaru is going to be trying to do. Briefly, trading pieces off is good for Hikaru, and that's what he starts to do.
You can see from the evaluation bar this Stockfish says he is slightly better.
Then he plays Bg5 which looks like an easy way for force a pair of bishops off, continuing the plan. But look at the evaluation bar! It quickly swings from 0.2 in favor of white to 1.7 in favor of black. But black can only realize that advantage by playing RxN, a move that Fabiano did not even consider. He went on to lose the game.
A prearranged signal from a confederate that meant "Hikaru just made a game changing blunder" would very likely have resulted in Fabiano seeing RxN. It's a move that many would spot if they were given the position as a puzzle and so knew there was a tactic somewhere.
[1] https://youtu.be/acjI2KqQ0gI?si=qkfkL6i53UDcBOQd&t=752