A hydraulically controllable couple, comprising: an input shaft coupled to rotate one of a body defining a chamber, the body in fluid communication with an inlet, and a rotating group disposed in the chamber; an output shaft coupled to rotate the other of the body and the rotating group, wherein the rotating group comprises: a rotor to rotate around an axis inside the chamber, with the rotor defining a first slot extending parallel to the axis along an exterior of the rotor and opening to the chamber, and a second slot opposite the first and opening to the chamber, the rotor further defining a retainer passage in fluid communication with the first slot; a first vane disposed in the first slot; a second vane disposed in the second slot; and a hydraulically controlled retainer disposed in a retainer passage to retain the first vane in a retracted vane mode of operation and to release the first vane in a vane extended mode of operation in which the first vane and the second vane extend to meet the body to hydraulically work fluid when the first vane and the second vane are moved with respect to the body; and an adjustable pressure relief valve (714, 1218) to adjust the maximum amount of torque transferred from the input shaft to the output shaft; wherein the input shaft and the output shaft are coupled to rotate together in the vane extended mode of operation, and the input shaft and output shaft are free to rotate with respect to one another in the vane retracted mode of operation. an input shaft coupled to rotate one of a body defining a chamber, the body in fluid communication with an inlet, and a rotating group disposed in the chamber; an output shaft coupled to rotate the other of the body and the rotating group, wherein the rotating group comprises: a rotor to rotate around an axis inside the chamber, with the rotor defining a first slot extending parallel to the axis along an exterior of the rotor and opening to the chamber, and a second slot opposite the first and opening to the chamber, the rotor further defining a retainer passage in fluid communication with the first slot; a first vane disposed in the first slot; a second vane disposed in the second slot; and a hydraulically controlled retainer disposed in a retainer passage to retain the first vane in a retracted vane mode of operation and to release the first vane in a vane extended mode of operation in which the first vane and the second vane extend to meet the body to hydraulically work fluid when the first vane and the second vane are moved with respect to the body; and a rotor to rotate around an axis inside the chamber, with the rotor defining a first slot extending parallel to the axis along an exterior of the rotor and opening to the chamber, and a second slot opposite the first and opening to the chamber, the rotor further defining a retainer passage in fluid communication with the first slot; a first vane disposed in the first slot; a second vane disposed in the second slot; and a hydraulically controlled retainer disposed in a retainer passage to retain the first vane in a retracted vane mode of operation and to release the first vane in a vane extended mode of operation in which the first vane and the second vane extend to meet the body to hydraulically work fluid when the first vane and the second vane are moved with respect to the body; and an adjustable pressure relief valve (714, 1218) to adjust the maximum amount of torque transferred from the input shaft to the output shaft; wherein the input shaft and the output shaft are coupled to rotate together in the vane extended mode of operation, and the input shaft and output shaft are free to rotate with respect to one another in the vane retracted mode of operation.