As shown below, the level of the line is protected by the node. When the point voltage exceeds Vcc+0.7V, the diode conduction is above; and when the voltage of the point is less than -0.7V, the following diode conduction is switched on. Therefore, the voltage is clamped between Vcc+0.7V - -0.7V.

For the normal diode, the forward resistance is about thousands of Europe, the negative resistance is several hundred kilohms (general should be more than 200 thousand euro), and MOS tube in 10M - 1000G resistance in Europe, so the diode resistance is much smaller than the resistance fet. If the voltage is too high, higher than Vcc+Vd (diode voltage), the diode conduction, the output voltage clamp at Vcc+Vd; if the voltage is too low, less than 0-Vd (diode voltage), the diode conduction, the output voltage clamp at -Vd.


From the electronic reading:
　　TVS diode selection procedures and type selection should pay attention to the problem
　　The typical application of TVS diode in the circuit design
　　Characteristics and main parameters of TVS diode
　　How to distinguish unidirectional and bidirectional TVS diodes?