A turbine is provided, comprising a housing having a fluid inlet and a fluid outlet, each in communication with an internal chamber formed within the housing. A rotor is mounted for rotation within the internal chamber. At least one paddle is pivotally mounted to the rotor for pivoting between an extended position in which the paddle extends substantially radially from the rotor for receiving pressure exerted by fluid moving through the housing and causing the rotor to rotate, and a retracted position in which the paddle does not extend from the rotor. A cam is provided in the internal chamber for moving the paddle from the extended position to the retracted position when, during rotation of the rotor, the paddle moves past the cam in the extended position. Magnetic biasing means may be used to bias the paddle in the extended or retracted position.