<p>In this study, we propose a novel torsional-spring mechanism employing a dual extension spring configuration to isolate forced torsional vibrations in mechanical systems. Unlike traditional negative stiffness mechanisms that rely on compression springs, the proposed mechanism avoids drawbacks inherently associated with compression springs, such as spring buckling, structural complexity, and large size, by employing extension springs. The mechanism transmits the average or slowly modulated torque components while isolating unwanted high-frequency pulsating components. When subjected to torque inputs, the two extension springs generate equal-magnitude forces in opposite directions, producing a force couple. The arm length of this force couple decreases as angular displacement increases, imparting stiffness-softening characteristics to the isolation system. As the average torque level rises, the shorter couple arm length, adapted to the adjusted nominal state, implies that the increased average torque is successfully transmitted while enhancing the isolation of pulsating torque owing to the softened stiffness. Because the amplitude of pulsating torque is often proportional to the average torque, this adaptive enhancement can be beneficial. To demonstrate the torsional stiffness-softening behavior and the adaptive isolation performance, we develop a simplified but accurate mathematical model incorporating a modified quintic term in a truncated Taylor series expansion. Through harmonic balance and bifurcation analyses, we propose a rigorous design approach to identify permissible specifications of average and pulsating torques, thereby preventing dynamic instabilities originating from stiffness nonlinearity and offering effective vibration isolation.</p>