If the power they are making is not being used up, their generators experience less load (they become easier to spin) and they start to speed up above the 50/60Hz setpoint. They monitor this and lower the energy input to the turbine spinning the generator. The opposite happens when they are making too little power. Note that this frequency shifting affects all generators connected to the same grid, so it might not need every power plant to change up their output to balance it. They communicate with each other and the plant(s) which can most economically ramp up/down at the moment to balance the system do so.
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