Reaction type turbines develop torque by reacting to the fluid's pressure or weight. The pressure of the fluid changes as it passes through the turbine rotor blades. These blades have a more internal efficiency, due to lower mean velocities and a more convergent character of its steam flow. A pressure casement is needed to contain the working fluid as it acts on the turbine stage or the turbine must be fully immersed in the fluid flow (wind turbines). The casing contains and directs the working fluid and, for water turbines, maintains the suction imparted by the draft tube. Most of the steam turbines use this concept.