It's not the heat that the CdS cell is responding to, it's the free electrons that are knocked loose from the crystalline structure by the light that allows the electrical current to flow.Therefore a flash of extra light can have an effect on later exposure to light briefly. It's like there is a threshold level of light to get the current flowing, similar to the way film responds to light. How much effect this is will need to be experimented with, though textbooks say CdS cell do have a "memory" that fades over time.