Steady, low voltage on pre-cat O2 sensor while idling, but voltage increases upon revving

To test my pre-cat O2 sensor, I unplugged its connector from the ECU and stuck a voltmeter’s probes into said connector.

When I let my engine idle, my voltmeter shows a steady low voltage, oscillating between 0.1 and 0.2 volts.

Shortly after I revv my engine, my voltmeter reading goes up to 0.8 to 0.9, then slowly declines back to 0.1 to 0.2.

What does the above mean for the engine and O2 sensor?