To be honest with you, I could never fiqure out why the high speed comes on when the sensor is disconnected, but I have noticed that this test does not allways work. It seems to work at certain temps and not others.
The resistance value drop is calibrated to turn on a switching device
[ maybe a gate or SCR, etc, - not sure] in the ECU. From there the relay gets the signal. It may be that the disconnect spikes the correct trip - maybe someone knows ???
Anyway, as a precaution ,I always use a 1/4 watt, 180 ohm resistor so I am sure of not screwing with the ecu with a direct short or open circuit.
The 180 is beyond the res/temp range for cut-in, so that seems to work well for the high speed test....