This paper presents an active dual-mode twisting actuation mechanism, which is an automatic transmission system based on twisted string actuation. This mechanism produces two different transmission ratios by adjusting the radius of a twisted string. Automatic mode switching between the two transmission ratios is implemented using an active control mechanism. Then, the effect of the proposed mechanism is derived via kinetic and kinematic analyses. Mathematical models of the twisted string are derived and a control algorithm for the mode transition is proposed. The performance of the proposed mechanism is experimentally verified using a prototype of the active dual-mode twisting actuator.