This paper presents a novel mechanism that allows us to develop energy efficient variable stiffness actuators that can provide rapid and wide range stiffness modulation in practice. Although infinite-range stiffness modulation can be achieved in principal, the rigid mode of the actuator is impractical due to manufacturing limitations. The stiffness of the actuator is modulated by changing the effective length of a group of leaf springs through a novel mechanism. The nonlinear behaviours of the leaf springs enable rapid and wide-range stiffness modulation, which can provide many benefits in advanced robotic applications such as human-robot collaboration and safe physical robot environment interaction. The leaf springs also allow us to develop energy efficient variable stiffness actuators. The servo system that modulates the mechanical stiffness does not drain power to keep the stiffness of the actuator fixed at equilibrium positions. Therefore, the proposed variable stiffness actuator is more energy efficient than the conventional antagonistic actuators that constantly drain power for stiffness modulation. When the stiffness of the actuator is changed at equilibrium positions, the power drain of the stiffness modulation motor due to inertial and frictional disturbances is negligible. The disturbances of the stiffness modulation motor exerted by the novel mechanism increases as the deflection angle of the output link increases. Nevertheless, the power drain by the stiffness modulation motor is always bounded at non-equilibrium positions. A variable stiffness actuator can be easily developed by placing the novel stiffness modulation mechanism onto a rigid actuator's output. Simulations and experiments are presented to verify the proposed novel stiffness modulation mechanism.