Imagine one ship going upwards in a lock, while another goes down. Ships and locks are the same size. To start with, there is a difference in the water levels

Imagine one ship going upwards in a lock, while another goes down. Ships and locks are the same size. To start with, there is a difference in the water levels - and this can be put through a generator to generate electricity as the water flows from one lock to the other. When the water has moved half way, the levels are equal. You can now use electricity to pump the rest of the way - and theoretically, assuming lossless pumps and generators, you use the same amount you generated earlier. Unfortunately, low-head generators tend to be inefficient and expensive. Variable head generators are even harder to design to be efficient.