This paper presents a fully classical, nonlinear, closed-form solution for the dynamics of the spin magnetic dipole moment in magnetic resonance imaging (MRI) under continuous-wave (CW) magnetic excitation. Unlike the commonly used linearized small-signal approaches or the quantum operator-based Rabi solution, the analysis here is developed entirely within a classical framework using rotating-frame vector calculus. The derived equations capture the complete angular evolution of the spin vector, provide insight into the reactive energy exchange with the environment, and show that no net energy transfer occurs over a full excitation cycle. This behavior is analogous to that of reactive circuit elements and has fundamental similarities with the stimulated emission radiation in optical Lasers. To the best of the author’s knowledge, this represents the first such nonlinear large-signal analysis based solely on classical physics.