I have no idea about the placements on the temple and left arm, but left-temple / right-temple placements are basically how electroconvulsive ("electroshock") therapy works (in that case, the current is used to generate seizures).

I'm not a doctor, but as far as I understand, the human body is highly conductive -- and there are a number of "wires" and conductive fluids running into and out of the brain. The entire nervous system is, in a very rough sense, electrochemical circuitry.