According to the wiring diagram, the center terminal is common. So assuming (danger!) you have the multimeter set to Ohms (resistance), with one test lead on the center and the other test lead on one of the other two terminals, you will have high resistance shown and by reversing the leads you will have much lower resistance. Keeping one lead on the center terminal and the other lead on the other outside terminal, you should have the same results. There are actually two diodes in that little gizmo. A diode is essentially a one-way electrical valve, permitting current to flow in one direction a blocking current in the other.