How does BZX84C7V5 work in the circuit?

I am researching the zener diode and circuit these days.

The zener diode breakdown voltage (stable voltage) test needs a small current voltage source which is higher than the breakdown voltage. The tester uses a 555 chip to form an oscillator, and then the output pulse is boosted to more than 100 volts for testing.

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Using a single 555 timer IC and a small transformer to generate high voltage, the circuit will test the zener diode with a voltage rating up to 50VDC. The 555 timer is used in the unsteady mode, and the output of pin 3 drives a small audio transformer, such as LT700. This has the primary impedance of 1K and the secondary impedance of 8 ohms. It is used to reverse the AC voltage to about 120 volts. It is rectified by a 1N4004 diode and filtered by a 2.2U capacitor to get about 150 volts DC. Connect the measured zener diode with the DC voltage meter of the multimeter, as shown in the figure. Load current switches enable zener diodes to switch at 1 or 2 Ma DC.

I am researching the zener diode--BZX84C7V5,( BZX84C7V5 PDF),and I would like to know that what they're used to work at constant voltage, but I don't know how they work. Actually, I kind of get that too, I know that voltage is constant until breakdown occurs (avalanche breakdown), but I don't get why a minimum current is needed to maintain a constant voltage, which is what the textbooks say. Why is there a minimum? I thought any current below the breakdown voltage was constant. I don't see why the voltage would vary through the diodes if it's below a set amount of current, yet remain constant between voltages x-y?

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