The Zener lets overvoltage drain tp ground, the signal diode to prevent the voltage reversal when the piezo is let up from getting to the FET?
I used 2 diodes per piezo lead to keep the - to ground. Piezos are bi-polar between squeeze and release.
I view piezo spikes as thin and figure that between diodes and a cap the spike can be flattened to a pulse.
Maybe I need to learn more about piezo output....