A method comprising: selectively transmitting torque and rotation along a drill string that extends within a borehole and comprises multiple drill string sections connected end-to-end; incorporating a pair of transmission modifier devices at longitudinally spaced positions in the drill string such that an intermediate portion defined between the pair of transmission modifier devices comprises multiple drill string sections; using each transmission modifier device to transmit drill string torque and rotation between adjacent drill string sections at opposite ends of the respective transmission modifier device; and selectively switching each transmission modifier device between a disengaged mode in which the transmission modifier device transmits the drill string torque and rotation substantially unchanged, and an engaged mode in which the transmission modifier device modifies the drill string torque and rotation during transmission, to deliver output rotation at a modified output speed, characterized in that each transmission modifier device includes a transmission mechanism comprising a planetary gear system drivingly interconnecting an input member (205,202) and an output member (202,205) of a respective transmission modifier device, the method further comprising: disposing the transmission mechanism to the engaged mode by rotationally anchoring a ring gear member of the planetary gear system to a formation through which the borehole extends; and disposing the transmission mechanism to the disengaged mode by releasing rotational anchoring of the ring gear member to the formation, while rotationally keying at least a sun gear member or a planet gear carrier member to the ring gear member. selectively transmitting torque and rotation along a drill string that extends within a borehole and comprises multiple drill string sections connected end-to-end; incorporating a pair of transmission modifier devices at longitudinally spaced positions in the drill string such that an intermediate portion defined between the pair of transmission modifier devices comprises multiple drill string sections; using each transmission modifier device to transmit drill string torque and rotation between adjacent drill string sections at opposite ends of the respective transmission modifier device; and selectively switching each transmission modifier device between a disengaged mode in which the transmission modifier device transmits the drill string torque and rotation substantially unchanged, and an engaged mode in which the transmission modifier device modifies the drill string torque and rotation during transmission, to deliver output rotation at a modified output speed, a disengaged mode in which the transmission modifier device transmits the drill string torque and rotation substantially unchanged, and an engaged mode in which the transmission modifier device modifies the drill string torque and rotation during transmission, to deliver output rotation at a modified output speed, characterized in that each transmission modifier device includes a transmission mechanism comprising a planetary gear system drivingly interconnecting an input member (205,202) and an output member (202,205) of a respective transmission modifier device, the method further comprising: disposing the transmission mechanism to the engaged mode by rotationally anchoring a ring gear member of the planetary gear system to a formation through which the borehole extends; and disposing the transmission mechanism to the disengaged mode by releasing rotational anchoring of the ring gear member to the formation, while rotationally keying at least a sun gear member or a planet gear carrier member to the ring gear member. disposing the transmission mechanism to the engaged mode by rotationally anchoring a ring gear member of the planetary gear system to a formation through which the borehole extends; and disposing the transmission mechanism to the disengaged mode by releasing rotational anchoring of the ring gear member to the formation, while rotationally keying at least a sun gear member or a planet gear carrier member to the ring gear member.