I thought that there were only 3 units. Regardless, if they can pump or generate, it means that at night when demand is low, they can theoretically run electric pumps to pump water back up into the reservoir. That provides more stored energy for daytime peaks, and evens out the nighttime load as well..
Look up pumped storage plants. We have one nearby called Raccoon Mountain. There are several around the country.
Yes, I understand pumped storage, there is a huge one near here.
If that is what they are trying to say, then
the description they give in the caption is poorly written.