I used both channels from the ET-312. The black lead is a common "ground" that is the second connection for both channels - they are spliced together in my cable. Then connected to the copper tube at the scrotum area. Channel A "hot" is then going to head clamp and the Channel B "hot" is going to a clamp somewhere in the middle of the cock. The stimulation is then felt between the each channel and the scrotum connection. The sensations are incredible, hope that explains