In this work, we present a complete mechanistic picture to understand a single molecule strongly coupled to a cavity and how VSC enhances the rate constant. In particular, we investigate how cavity lifetime τ c influences the rate constants and derive a new analytic expression of the VSC rate constant under the lossless regime, based on a mechanistic observation that the rate-limiting step is the photonic excitation and the subsequent excitation transfer between photonic and vibrational DOFs. The resulting analytic rate theory, denoted as kVSC (see Eq. 17), successfully described the VSC rate constant in the lossless regime and is in excellent agreement with the numerically exact results. Not only it predicts the correct resonance behavior at ω c = ω 0 , but also gives a clear explanation for the intimate connection between the VSC-modified rate constant and the optical lineshape A ν (ω -ω 0 ) (Eq. 14). To the best of our knowledge, this is the first analytic theory that explains the close connection between rate constant changes and lineshape of the vibrations.