  Based on the substantial difference in the response time for the resonant and
background partitions at stepwise variations of the exiting signal, a simple
exactly integrable model describing the dynamic Fano resonance (DFRs) is
proposed. The model does not have any fitting parameters, may include any
number of resonant partitions and exhibits high accuracy. It is shown that at
the point of the destructive interference any sharp variation of the amplitude
of the excitation (no matter an increase or a decrease) gives rise to
pronounced "flashes" in the intensity of the output signal. In particular, the
flash should appear behind the trailing edge of the exciting pulse, when the
excitation is already over. The model is applied to explain the DFRs at the
light scattering by a dielectric cylinder with two resonant modes excited
simultaneously and exhibits the excellent agreement with the results of the
direct numerical integration of the Maxwell equations.
