- Gamma ray bursts (GRB's) often feature subpulses that have a distinctively asymmetric profile - they rise quickly and decay much more slowly, while their spectrum softens slightly with observer time. It is suggested that these subpulses are caused by slow baryonic clouds embedded within a primary -ray beam, which scatter the -radiation into our line of sight as they accelerate. Good quantitative agreement is obtained with observed light curves and spectral evolution. The kinetic energy that the baryonic component of GRB jets receives from the primary -radiation is predicted to be about equal to the amount of -radiation that is scattered, consistent with observations of afterglow. Several other observational consequences are briefly discussed. The possibility is raised that the time scale of short GRB is established by radiative acceleration and/or baryon injection rather than the time scale of the central engine.