A nonlinear feedback mechanism for the generation of Jovian S bursts is reexamined in the light of recent observations of S burst phase coherence. In this model, phase-coherent radiation is generated by phase-bunched electrons within interaction regions drifting along the Io flux tube. It is shown that the same mechanism, operating in two separate regimes, can produce both constant-frequency (zero-frequency-drift) and rapid-frequency-drift emissions. Evaluation of S burst drift rates reveals that several unexplained properties of S bursts, namely, their complex morphologies and interactions with narrowband L bursts, are consistent with a phase-bunching model.