Light leads to conformational changes in rhodopsin, which in turn leads to the activation of transducin. Transducin activates phosphodiesterase, which results in the breakdown of cyclic guanosine monophosphate (cGMP). The intensity of the flash response is directly proportional to the number of transducin activated.
^Lerea CL, Somers DE, Hurley JB, Klock IB, Bunt-Milam AH (October 1986). "Identification of specific transducin alpha subunits in retinal rod and cone photoreceptors". Science. 234 (4772): 77–80. doi:10.1126/science.3529395. PMID3529395.