Steam turbines use the pressure energy of steam to power rotation of a shaft. There are two main types: impulse turbines, where steam jet kinetic energy changes the turbine blades' momentum; and reaction turbines, where continuous steam pressure drop over fixed and moving blades provides rotational force. Compounding involves using multiple stages to reduce turbine speed for practical use, through either velocity compounding by absorbing steam kinetic energy in stages, pressure compounding by expanding steam pressure in nozzles, or a combination of both approaches in pressure-velocity compounding turbines.