Piezoelectricity is electricity generated when substances are subjected to pressure/impact. Common uses are in the electric 'spark' on a stove or cigarette lighter (the push-button type, not the kind with the flint-wheel), or in guitar pickups. Knowing this leads to GamerwithnoGame's answer: In the case of this mod, the impact is from the rain on the solar panels.
In the real world the effect would be caused by Atmospheric pressure (Warm air with no moisture is lighter).
Forgive me for being a pedant, but 99% of guitar pickups are electromagnetic. The rest
are Piezoelectric, such as the stick-on types for accoustic guitars with nylon strings, but there was one I used in a guitar that was actually a bridge replacement with a separate pickup in each string saddle.
It could produce a near perfect accoustic tone (before effects) from the grungiest of rock electric guitars, but the main reason I used it in my builds was to isolate each string's vibrations and feed them separately into a 6-channel MIDI pickup to be equalized and otherwise processed to drive a synth.
Piezos are interesting:
1) They are pressure sensitive, producing a small voltage with pressure variance. (vibration counts as pressure variance, BtW)
2) If you apply a small voltage they vibrate, the resulting frequency depending on the size and shape of the crystal.
The second trait was the first to be widely used, to tune radios before variable oscillators and then Phase-locked-loops were invented. This trait still functions as the base frequency of almost all computers, usually 100MHz which is then multiplied to the GHz numbers we know now.
