Rectifier Circuit
When the input current flows in the positive direction, current flows in at A, through the diode in arm AB, and out at B. Current flows from the output in at D, through the diode in arm DC and out at the input at C.
When the input current flows in the negative direction, current flows from the diode circuit into the input at A, having come through the diode in arm DA, and before this, the output terminal at D. Current flows from the input at C through the diode in arm CB at out at B.