When water flows inside of a pipe, there's a portion of it that flows right through, then there's a portion of it that just sticks to the pipe wall: that's called the boundary region.
Now you can try to pump a coolant faster, and minimize this boundary layer, but it's still going to be there, and you'll waste a lot of pumping power to minimize it.
So when you shoot the water, you're essentially "attacking" the boundary layer straight on, with minimal flow.
The concept behind Cascade is detailed
here.