Aqueducts extends a single source block of water much further than the standard distance. So long as the aqueduct blocks are placed in a path, the water will continue the flow.
You can extend the flow downward so long as the aqueduct blocks are only a single block lower each level. And the flow can be guided around corners as well.
This mod essentially extends that single source block without needing to add more water to the flow. This allows you to use that flow for things like farming or bottling it.
And because this makes the entire flow just one long source block, you can stop it at any point and it will drain after the stoppage point.