gradient

If a fish is brave enough or gets pushed far enough into that voltage gradient, it will stun them.

It's pulsed into the water very fast to create a voltage gradient in the water, like a speed hump that you would see on a road.

You can also note that the drain actually increases the hydraulic gradient by shortening the flow paths, so there’s actually more seepage happening than there would be without the drain.

Besides the permeability of the soil, the only other factor controlling the speed water flows is the hydraulic gradient, which consists of the difference in pressure over the length of a flow path.