Recent experiments show that a dielectric-barrier-discharge can fundamentally change the character of a hydrogen diffusion flame, far more so than for a corresponding inert-fluid actuation. The stand burner flame surface deforms from roughly conical to nearly flat, and light emissions increase. We develop a simulation model to analyze the mechanisms that underlie these changes, and reproduce key observations. The main mechanisms are body forces due to charge sheaths, with radicals produced by plasma excitation playing a secondary role for the present conditions. The non-actuated flame flickers at approximately 10 Hz, in good agreement with the experiments. As the DBD voltage is increased, the flame flattens and oscillations decrease, eventually ceasing above a threshold value. In the fully flattened case, the stoichiometric surface lies flat across the fuel orifice, and the flame temperature exceeds the adiabatic flame value. When calibrated against an independent air-only experiments, a linearized plasma sheath for model reproduces the main features of the experiments and provides a good estimate for the thresh-old flattening potential. The model also anticipates faster flowing regimes in which radical production by the plasma is anticipate to become more important.