According to IEEE Spectrum:
“A handful of athletes competing at the Summer Olympic Games in Rio next week will arrive having tried to boost their performance using an unconventional (and not-yet-banned) technology: brain stimulation. The technique, called transcranial direct current stimulation (tDCS), involves channeling a tiny current through specific regions of the brain, making neurons in that area more likely to fire.”
The athletes are using the Halo tDCS device. Here is the promotional video:
Before you run out and buy one of these devices, I suggest you read “An open letter concerning do-it-yourself users of transcranial direct current stimulation” published in The Annals of Neurology.