Complex sentence: The may also be used for pumped storage; a reservoir is filled by the turbine (acting as a pump) during low power demand, and then reversed and used to generate power during peak demand.
Split into simpler sentences: The may also be used for pumped storage, where the machinery is reversed and water is pumped. A reservoir is filled by the turbine (acting as a pump) during low power demand, and then reversed and used to generate power during peak demand.