pleurosternal muscles are thought to tension the thorax. when the muscle contracts the thorax becomes stiffer and thus increasing wing beat frequency because this ultimately changes the resonant properties of the thorax.
references
Dickinson, M. H., & Tu, M. S. (1997). The Function of Dipteran Flight Muscle. Comparative Biochemistry and Physiology Part A: Physiology, 116(3), 223–238. https://doi.org/10.1016/S0300-9629(96)00162-4