Though there's no answer in the video, the classic demonstration of TMS (Transcranial Magnetic Stimulation) you'll find in an intro to brain imaging class is to trigger sections of brain containing the motor neurons. This creates involuntary movement in the muscles of your body, rather than any kind of information you would process willfully act upon. Given the mention of placing the TMS paddle on the opposite hemisphere of the brain from the subject's arm, it's plausible that they are actually just aiming at the motor neurons.