Reaction turbines are seemingly innoculous pieces of equipment. A working fluid (steam, exhaust gas or water) causes the turbine rotor to spin, converting the kinetic energy of the fluid into mechanical energy. Simple enough. At first glance, all turbines would seem to operate due to direct fluid pressure on the blades, a la impulse turbine.

However, things aren't quite that simple. Enter Newton's Third Law of Motion.

Good 'ol Isaac figured out that for every action, there is an equal and opposite reaction. This principle is what drives a reaction turbine.

As the working fluid flows through the turbine blades, it is deflected off the blades at a different angle than which it first hit the blade. This change in direction of the working fluid causes a force to be exerted on the turbine in the tangent direction, causing it to spin. Some reaction turbines have their nozzles attached to the rotor. One of the earliest was built by Hero of Alexandria waaay back in 75 AD.

The (minute) difference between the impulse turbine and reaction turbine is much more easily shown with the following example. Imagine you are sitting in a shopping cart and holding onto a firehose nozzle. When the pump truck turns the pump up to eleven, you'll get pushed in the opposite direction as the nozzle is pointed. Hence, you are being driven by reactive forces. To get driven by impulsive forces, you would have to get sprayed with the firehose.

In hydroelectric power generation, reaction turbines are most suited for medium to low head. Examples of reaction turbines are Francis, Parsons and propeller turbines.

Most turbines operate as a combination of impulse and reaction modes. The distinction between the two is used to show how it primarily receives power from the working fluid.