Communication could be done entirely invisibly from an outsiders perspective. For example, embedded electrodes sensing when one clenched their jaw muscles.

My brother and I did this magic trick and convinced our grade school we had ESP... the mark tells brother a secret number. I put my hands on brothers head and without communicating I tell what the secret number was. It was just the number of times he clenched his jaw.

Put them in a room without window or any exterior interactions, 4 cameras in each corner, wifi/radio signal jammer, 10 minutes delay with livestream, I would honestly pay respect to Hans if he can still cheat in those conditions

Is wifi even necessary? Could one embed a chip which does chess inference somewhere in one's body?

If one was willing to go as far as devices implanted into the body, it's somewhat easy to imagine inputs via normal-looking fidgeting: toe taps, even perhaps clenching some body part.

While not trivial, it doesn't feel like the hardest part of that particular engineering challenge.

This being HN, I'm surprised nobody has mentioned that they could build an OpenCV system to watch a match in progress over a weekend.

At GM levels the matches are typically broadcast live and/or have an audience, so it would only require an observer to input the moves.

This can be counteracted by broadcasting with a delay, something I believe they already do. It may be the case that the delay needs to be increased.

Chess turns can take 20+ minutes sometimes depending on the time limit of the game, so that would be quite a delay, and then you would still have a problem with a live audience.

How about a 'turn-delay'... the broadcast field is always three turns delayed rather than a static time delay

Well if there's 3 rapid fire exchanges (trading Queens & Rooks for example) lasting 10 seconds total, followed by a 20 min move, then the broadcast would have to freeze frame on the 1st move in the rapid fire sequence, so as to stay 3 moves behind, with the last 2 "hidden" moves evident to all that the pieces will be traded.

Then you again require communication to the players, which a Faraday cage would solve.

Arduino you ain't cheatin'? :)