[cupofjoakim]

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.

[KingMob]

The shoe insert has actually been built, though I haven't heard if it, or something similar, has been used in real cheating yet.

It's a shoe-to-Pi device called Sockfish (a pun on the Stockfish chess engine): https://incoherency.co.uk/blog/stories/sockfish.html

[Hanschri]

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.

[kareemsabri]

Wow, this goes deep!

[rendall]

Well done!

[moralestapia]

And all of those are extremely easy to prevent/spot in a tournament setting. Carlsen is just throwing a tantrum here, IMO. Sore loser.