How is this possible? From an engineering perspective, how are power stations designed to handle continually varying loads? It’s all about speed control of generator. Any increase demand in load results in momentarily reduction in speed. Observation in speed reduction required immediate action. System increases steam injection by controlling valve in case of thermal power plant and same thing happens in other cases also. This is also why we need to maintain the constant frequency and any large variation in frequency may results complete shout-down blackout (due to protections system in picture). So monitoring speed and controlling fuel injection are critical part of power system stability & reliability. So when you turn on or turn off the light, power station immediately operates steam valve.