Reverse breakdown is what happens when a reverse-biased Diode is pushed past a critical reverse voltage: instead of blocking, it suddenly conducts heavily in the reverse direction. The voltage at which this onset occurs is the breakdown voltage .
What actually happens
In Reverse bias the standard story is “essentially no current flows” — only the tiny Reverse saturation current . That holds only up to a point. As the reverse voltage climbs, the electric field across the depletion region of the PN junction grows. Past a critical field the junction breaks down by one of two mechanisms: in the avalanche mechanism, carriers accelerated by the field gain enough energy to knock other electrons free on impact, which knock more free, in a multiplying chain; in the Zener (tunnelling) mechanism, which dominates at low breakdown voltages, the field is strong enough that electrons tunnel directly across the junction. [The mechanism detail — avalanche vs. tunnelling — is background from general knowledge; the source PDF only describes the observable behaviour.] Either way the current rises almost vertically with no further increase in voltage.
Reverse current stays at the small saturation value until breakdown , then rises sharply.
Destructive vs. engineered
Breakdown itself does not destroy a junction — heat does. The power dissipated in breakdown is , and with the steep curve a small voltage overshoot causes a huge current. For an ordinary signal or rectifier diode, with no external resistor to limit the current, exceeding runs the power dissipation away and cooks the device. That is why a diode’s reverse voltage rating (Peak inverse voltage) must never be exceeded in a rectifier.
But the breakdown region is flat in voltage — a large change in current barely moves the voltage. That is precisely the behaviour you want from a voltage reference. The Zener diode is a diode engineered to break down at a precisely controlled and, with a series current-limiting resistor keeping the dissipation safe, to operate reliably in this region. That is the basis of the Zener voltage regulator.