"The gain mechanism, which is analogous to an electromagnetic traveling-wave tube amplifer32, depends on the average interaction of the charge carriers with the piezoelectric electric field. This interaction controls the energy transfer from or to the acoustic wave, resulting in acoustic wave attenuation or amplification, respectively."
Source: https://www.nature.com/articles/s41467-021-22935-1
I was pleasantly surprised that it wasn't paywalled.