A method and apparatus according to which an output of a power generation system is controlled. In one embodiment, the power generation system includes a turbine and a feedback control system. The turbine includes a rotor to which a first portion of a power fluid is communicated, the first portion imparting torque to the rotor; a shaft to which the rotor is connected; a shroud extending circumferentially about the rotor and the shaft; and a bypass gap between the rotor and the shroud, through which a second portion of the power fluid is communicated. The feedback control system axially displaces the shroud relative to the rotor, thereby adjusting the size of the bypass gap and, consequently, the ratio of the first portion relative to the second portion.