There are many ways. You need outside help for everything though - you need someone to input the current board state into some kind of chess engine, and then relay the best possible move back to you somehow. There have been cases where someone has hidden a phone in the bathrooms to use for either communicating or inputting into a chess engine themselves.
In Niemanns case people have joked about vibrating anal beads communicating in morse code as well as shoe inserts.
You don't even have to relay the move to the player, simply giving a sign that this move is important can be enough for the player to understand they've got the edge and utilize that. It could be as simple as someone in view giving a sign with body language. Remember also that a professional chess player doesn't need to cheat every single move, cheating once in a game at the right move can be enough to put the game in your favor at such a high level.
In most cases players were using a smartphones on the toilet.
Generally there are some more options:
- using a small bluetooth headphone with external person telling you the moves
- a vibrating device on the body that vibrates you the moves
Adding to the posters above, there are headphones nowadays which look like a small stone and are put directly on the ear drum (yes, they need to be extracted by someone afterwards). They are completely invisible outside. They popular with students for cheating at exams for example.
Some tournaments allow you to take breaks away from the board. If you're at a venue with a balcony (like the St Louis chess club), someone could give you light signals from a parked car.
That's a relatively low tech solution, it gets crazier from there as other people are saying here.
With a computer. A simple way is to have a chess game on your machine playing at the most difficult setting. You enter your opponent's move, the computer plays a move in response which you then replicate in the game against your opponent.
at very high level you don't need a lot to turn the tables. a sign of when to act agressive/defensive could be enough. I can think of millions ways to give this sign.
Yes. This can't be overstated enough. Cheating doesn't require concealed computers or any sort of electronic communication at all.
People might scoff but honestly, chess players have worried about this sort of stuff for decades. The '78 world championship between Karpov and Korchnoi for example, is a lesson in this kind of cheating - I'm thinking of the yogurt controversy in particular.
I don't believe for a moment that Karpov or his team were cheating in '78 (at least not in that way) but the fact that they weren't is really the salient point - it is very difficult to ignore the idea that your opponent might be cheating once the possibility has been suggested.
That is to say, trust in your opponent is a very important element of competitive play.
In Niemanns case people have joked about vibrating anal beads communicating in morse code as well as shoe inserts.