The diode protection circuit shown in Fig. 1 is designed to protect the input of a circuit from voltages higher than the Vdd voltage rail (in this example the rail is considered 3.3V), and lower than ground. Circuits like this are commonly used in digital systems that are sensitive to over voltages, where they need the input voltage of a circuit to be kept within a specific range.
This circuit can be analyzed using the following simple model of a diode:
When the diode is reversed biased (like in Fig. 2a), the voltage at node A is higher than the voltage at node B. Essentially, this means the voltage drop across the diode does not match the diode's polarity. A reverse biased diode can be thought of as an open switch because it stops all current from flowing through. A diode that is forward biased (as shown in Fig. 2b) has voltage at Node A which is lower than node B. This means the voltage drop across the diode does match the diode's polarity. In this case the diode can be thought of as a closed switch or short which allows current to pass through.
When Vin is in state 1, the voltage of Vin is between 3.3V and 0V, as displayed in Fig. 3. Both diodes are reversed biased and act like switches in the open position. In this state the voltage across the load resistor is exactly equal to Vin, which is fine because this is our ideal operating region.
In state 2, Vin is greater than 3.3V. In this case the diode tied to 3.3V is no longer reverse biased. So there is a short between 3.3V and node A. This has the effect of detouring the over voltage current back to the power rail. Since most input signals (to circuits like this) have a very low current and short duration, the power rail has no problem dealing with the extra voltage. So overall, the output to node A is kept very close to the 3.3V rail voltage.
Finally in state 3, Vin is less than ground (i.e., a negative voltage spike). So the top diode is still reversed biased, but this time the bottom diode is forward biased. This now has the effect of shorting Vin to ground, so the overall output to node A is very close to 0V.
A circuit configuration like this is commonly used to protect the inputs of a circuit from ESD (Electro static discharge). Both ESD and the voltage from the piezoelectric element share similar properties in that they are both high voltage, low current, short duration signals. This means a circuit of this type would be ill-suited for protection against a constant overvoltage or high current signal.