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.
You use a chess engine to tell you the best move - you can run a chess engine on a modern phone that will easily best the world's top human chess players.
The simplest forms of this are things like: "play online, chess engine open in another window", "use your phone hiding in a bathroom cubicle" and "member of the audience follows your game with a chess engine and signals you somehow"
There are also rumoured to be very subtle ways of doing this - like playing unassisted for most of the game, but an engine providing 'flashes of genius' at one or two crucial moves of the game.
Major competitions have things like metal detectors and time-delay video feeds hoping to make cheating harder.
[1] Yes, the ribald jokes have already been made
I'm sure it would be a downer that I cheated but it would do them a favor by saying: "look, you cannot stop it. Time for something new".
It's not really cheating in NASCAR, but rather, "it wasn't in the rulebook".
"... But had I started cheating in a clever manner, I am convinced no one would notice. I would've just needed to cheat one or two times during the match, and I would not even need to be given moves, just the answer on which was way better. Or, here there is a possibility of winning and here you need to be more careful. That is all I would need in order to be almost invicible."
Or, more mundanely, bathroom breaks. https://www.nytimes.com/2024/10/16/sports/kirill-shevchenko-...
Your iPhone can reliably beat the best chess players in the world.
Before computers put an end to the practice, long games used to adjourn overnight. https://www.chess.com/terms/chess-adjournment
> During adjournments, players could count on the help of other strong masters, called seconds. These seconds would analyze the position and tell the player what they should play when the game resumed.
The way people cheat online is by running a chess engine that analyzes the state of the board in their web browser/app and suggests moves and/or gives a +/- rating reflecting the balance of the game. Sometimes people run it on another device like their phone to evade detection, but the low-effort ways are a browser extension or background app that monitors the screen. The major online chess platforms are constantly/daily banning significant amounts of people trying to cheat in this way.
Chess.com and Lichess catch these cheaters using a variety of methods, some of which are kept secret to make it harder for cheaters to circumvent them. One obvious way is to automatically compare people's moves to the top few engine moves and look for correlations, which is quite effective for, say, catching people who are low-rated but pull out the engine to help them win games occasionally. It's not that good for top-level chess because a Magnus or Hikaru or basically anyone in the top few hundred players can bang out a series of extremely accurate moves in a critical spot - that's why they're top chess players, they're extremely good. Engine analysis can still catch high-level cheaters, but it often takes manual effort to isolate moves that even a world-champion-class human would not have come up with, and offers grounds for suspicion and further investigation rather than certainty.
For titled events and tournaments, Chess.com has what's effectively a custom browser (Proctor) that surveils players during their games, capturing their screen and recording the mics and cameras that Chess.com requires high-level players to make available to show their environment while they play. This is obviously extremely onerous for players, but there's often money on the line and players do not want to play against cheaters either so they largely put up with the inconvenience and privacy loss.
Despite all of the above, high-level online cheating still happens and some of it is likely not caught.
Edit: More information on Proctor here: https://www.chess.com/proctor
Interesting; I thought I'd read that even the very best players only average ~90% accuracy, whereas the best engines average 99.something%?
Edit: Even lower-level cheated games are rarely 100% accurate for the whole game, cheaters usually mix in some bad or natural moves knowing that the engine will let them win anyways. That's why analysis is usually on critical sections, if someone normally plays with a 900 rating but spikes to 100% accuracy every time there's a critical move where other options lose, that's a strong suggestion they're cheating. One of the skills of a strong GM is sniffing out situations like that and being able to calculate a line of 'only moves' under pressure, so it's not nearly as surprising when they pull it off.
To compute accuracy, you compare the moves which are made during the game with the best moves suggested by the engine. So, the engine will evaluate itself 100%, given its settings are the same during game and during evaluation.
You get 99.9something% when you evaluate one strong engine by using another strong engine (they're mostly aligned, but may disagree in small details), or when the engine configuration during the evaluation is different from the configuration used in a game (e.g. engine is given more time to think).
Cheating is as simple as having somebody feed you chess engine moves from a nearby laptop running stockfish.