Impulse (Pelton / cross-flow) turbines use the kinetic energy (i.e. the velocity) of the water. The exchange of energy between the water flow and the blades takes place under atmospheric pressure. The wheel is placed above the surface of the water and revolves through the air. Impulse turbines are primarily used for high head applications.
Reaction (Francis, Kaplan) turbines use the kinematic as well as the potential energy of the moving water. Their design permits the creation of a "whirlwind". The blades propel the water parallel to the revolving axis and thus convert the energy of the whirlwind into mechanical force.
The generator then converts this into electricity. Both synchronous and asynchronous machines with variable speeds can be used. The speed is determined by the turbine, and in some circumstances a gear mechanism may be required.