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Magnus didn´t provide any proof, also the officials from that tournament couldn´t find anything. But technology today allows devices which would not be detected with the "Walmart" scanners used. So I guess for future events controls will have to be much sharper (like taking shoes off, radiation jamming in the playing hall, etc.).

But what exactly are they doing to cheat?

Like how are they being told the ideal move to make?

There are e.g. vibrating devices that could be inserted into the rectum. Not that this guy is telling the truth, but just as an idea:

Yep, wish I didn't ask haha.