Backdrivability describes the ability of a robot joint reducer to passively transmit external power during interaction with the environment, and is an important factor affecting the accuracy of robot force feedback. The reverse starting torque is commonly used to characterize a reducer′s backdrivability. However, studies have shown that reverse starting torque represents only a special case of backdrivability, corresponding merely to the specific boundary condition in which the reducer transitions from rest to motion. Therefore, it cannot provide accurate torque information for robot joint feedback control in engineering applications. A comprehensive review of the research and application status of backdrivability in robot joint reducers indicates that most existing studies consider only the simple influences of —moment of inertia and friction, while neglecting the effects of gear ratio and operating speed. Moreover, prior work has focused on static testing and evaluation of reverse starting torque, without addressing the backdrivability of the reducer during motion. To overcome these limitations, this paper conducts an in-depth investigation of the reducer′s bidirectional transmission mechanism and defines the backdrive torque as a parameter to characterize backdrivability. Based on a deconstruction analysis of backdrivability, a backdrive torque model is established that incorporates gear ratio, moment of inertia, friction, and speed. Furthermore, a dynamic characterization and testing method for reducer backdrivability is proposed based on energy transmission, enabling quantitative evaluation of backdrivability under different speeds and operating conditions. The effects of these four parameters on reducer backdrivability are analyzed theoretically, and experimental studies verify the dynamic nature of reducer backdrivability, revealing the inadequacy of using a single reverse starting torque index to evaluate reducer backdrivability. Finally, the theoretical and practical value of reducer backdrivability are highlighted.