The more straightforward analogy with water is a pipe that is completely filled. Even if the water moves very slowly, the fact that it has started moving is immediately detectable as some of it will start spilling from the other end of the pipe right away. If you have a turbine installed at the other end, it will also start spinning right away. Moreover, if you make the pipe wide enough, even very slowly moving water will move a lot of volume in a short period of time, and thus transmit a significant amount of energy to the turbine.