How does ESD protection work with TVS diodes?
If you consider what the 8 kV ESD really is in terms of energy storage devices and resistors it'll look something like this: -



Picture from here.

The 150 pF capacitor is charged to +8,000 volts but, your target circuit doesn't actually receive that voltage directly; there is a 330 ohm resistor in the way. That means the peak current delivered can be up to 24.24 amps.

So, when that hits the circuit node called "pin 1", D2 is forward biased and takes the brunt of the 24 amp spike and passes it through to the 6 volt DC bus. If the capacitance on your DC bus is (say) 100 uF, the voltage starts rising at this rate: -

$$I = C\dfrac{dV}{dt}\Longrightarrow \dfrac{dV}{dt} = \dfrac{24.24\text{ A}}{100\text{ uF}} = 242,240 \text{  volts/sec}$$

But, the CR time for the pulse is only 330 x 150 E-12 = 50 ns and so, the voltage might rise only 12 mV in those 50 ns. Taking this to extremes you couldn't really see the voltage rising much more than 0.1 volts before the ESD pulse has evaporated to nothing.

But how much voltage does the diode drop when taking 24 amps? It'll be 1 or 2 volts depending on the diode so, the maximum voltage that might be seen on pin 1 could be as high as 8.1 volts AND D1 (the other diode) will remain reverse biased with no harm.

Of course, 8.1 volts might still do serious harm to your pin so, most folk would have a 1 kohm resistor to the outside world from where your diodes are (see R1 below). This then restricts the current to 8000/1330 amps = 6 amps. The knock on effect should be clear.

Even so, it doesn't guarantee the avoidance of circuit failure and, in addition to the 1 kohm mentioned above, another 1 kohm resistor is placed directly in series with the input pin. The 2nd resistor (R2) makes use of the chip's data sheet that would inform you what the maximum current is that could be injected without harm.



The ESD discharge can also be negative, in which case D1 takes the brunt of the current and passes it (more safely) to ground.

Also, if pin 1 is an output, you might not be able to live with a couple of kohm of extra resistors and different measures have to be taken.