Water flowing into the top of the turbine not only spins the wheel by its weight, but as it flows past the blades and into the middle of the turbine, its direction is changed. This extracts additional power via Newton's Third Law. In order to be more powerful than an over-shot wheel, the water is funneled into a venturi before meeting the turbine, thus flowing over the blades at high speed.
The system is very similar to the Pelton wheel in concept, but requires less engineering in the wheel itself. Construction of the ventruri is somewhat more complex than in a over-shot wheel, but by no means difficult. Yet the Banki turbine is often many times more powerful than a traditional wheel, generating the same power as a large Diesel engine.