Continuum theory is used to explain why stop-and-go oscillations in congested freeway traffic change their amplitudes when they encounter the vehicular merging and diverging maneuvers that take place near ramps. The theory describes how oscillations diminish in amplitude when they propagate past a queued (and unmetered) on-ramp and how they grow when they propagate past an off-ramp. The premise is that merging (diverging) flows change in response to freeway oscillations and that these changes in flow dampen (amplify) oscillations. The theory's descriptions are simple and rational; all its inputs and outputs are directly observable; and its predictions are shown to match real data. The theory is tested against real data collected over multiple days from congested merge and diverge sites with videos and inductive loop detectors. For merges, predictions are found to agree with observation to within 10%, and for diverges, to within 12%. The paper thus resolves in a simple way a puzzling traffic feature reported in previous studies.