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.