It has hysteresis.
If you read through Joerg's description of testing the circuit, you may see.
When adjusting the voltage it should have two different voltages where it switches. At the lower of the two voltages, the field turns on. The positive feedback through the 120k resistor causes the transistor base to see a different voltage depending on whether the field switch is open or closed. At the higher of the two voltages, the field turns off. The positive feedback makes the circuit lock into that state, until the battery voltage changes enough to overcome the hysteresis. The whole point of the design is to prevent oscillation. The field can only switch as fast as the battery voltage drifts up and down through the hysteresis window. The size of the hysteresis window and the rate of change of battery voltage in response to the switching of the field determine switching frequency.
But in testing, you can move the pot as slow as you want. If you leave it alone, the circuit shouldn't switch at all. It should only switch when the voltage passes one of the hysteresis set points.
And it works for low field current, but at higher currents something causes the small transistor to oscillate.