in this video at 4min 4sec https://youtu.be/CkGVMWK10qU?t=243 The guy receive a shock of a taser by touching a metal object that touches one probe, but why does the arc goes through his body ? I know that there must be an electric potential difference in order to have current flowing through the body, so where is the second potential ? At first I though it was the ground he touches with his feets but his contraption runs on a drill battery and it looks like it is on a wooden table. Any help ?
He still touches button with its contacts by the other hand. That button is residing in the low voltage circuit, which is connected to the ground. Circuit diagram is at 3:28 of the video.
And finally, arc is going to find the "shortest" way to get through to the ground; the resistance of the body is much less than resistance of the air, even if it looks like physical distance between contacts is much less than distance between his fingers of the right and left hand. So it is fairly logical that it diverted to him rather than continuing going through the air between the contacts.