Conceptually, they just use their electrical generators as electric motors that consume electricity to pump water back through the system, from the lower reservoir back to the upper reservoir.
Pumped hydropower is a technology based on pumping water into an upstream reservoir during off-peak or times when there is excess electricity generated from renewable energy sources and when electricity is needed, it is released through hydro turbines.
By doing this during periods of low electricity demand, they use excess electricity generation capacity elsewhere in the grid to store a head of water for future use. Then, during periods of peak electrical demand, not only these other electrical generators are online, but also a hydroelectric generator with a good water fall in the upper reservoir.
It's a way to balance peaks and troughs in electricity demand during the day
Grand Coulee Dam in Washington State was originally built for flood control and irrigation in the central Washington agricultural basin. Surplus electricity was only a minor consideration, but today it is the largest hydroelectric generator in North America. The project has two huge reservoirs: Lake Roosevelt, part of the Columbia River raised by the dam; and Banks Lake, 280 feet higher than Roosevelt, used to irrigate 670,000 acres of farmland. Water is pumped up to Banks Lake through huge pipes (see photo) by reversible engines that can also serve as generators. I don't believe there is currently any working agreement between Grand Coulee Dam and a very large wind turbine system in the same region, but the potential for energy storage using Banks Lake seems natural.
The first hydroelectric plant was built at Niagara Falls in Buffalo N Y by Nichola Tesla and Westinghouse. It was used to power the 1896 World's Fair. It provided alternating current from 22 miles away and helped change the world. Since AC can be transformed to a higher voltage and shut down for domestic use, it has become the standard in the world.
Related Post - Hydropower Definition (How it Work)
Thomas Edison believed that the power supply should only be direct direct current, not alternating current, but this turned out to be incorrect.
It is also very interesting that all electric vehicles now use AC motors and a controller because they are very efficient, can do REGENERATIVE braking and do not use brushes that wear out. America's fastest growing new car company uses the name TESLA because it uses AC motors and controllers.