  We consider a magnetar flare model for fast radio bursts (FRBs). We show that
millisecond burst of sufficient power can be generated by synchrotron maser
emission ignited at the reverse shock propagating through the weakly magnetized
material that forms the magnetar flare. If the maser emission is generated in
an anisotropic regime (due to the geometry of the production region or presence
of an intense external source of stimulating photons) the duration of the maser
flashes is similar to the magnetar flare duration even if the shock front
radius is large. Our scenario allows relaxing the requirements for several key
parameters: the magnetic field strength at the production site, luminosity of
the flare, and the production site bulk Lorentz factor. To check the
feasibility of this model, we study the statistical relation between powerful
magnetar flares and the rate of FRBs. The expected ratio is derived by
convoluting the redshift-dependent magnetar density with their flare luminosity
function above the energy limit determined by the FRB detection threshold. We
obtain that only a small fraction, $\sim10^{-5}$, of powerful magnetar flares
trigger FRBs. This ratio agrees surprisingly well with our estimates: we
obtained that $10\%$ of magnetars should be in the evolutionary phase suitable
for the production of FRBs, and only $10^{-4}$ of all flares are expected to be
weakly magnetized, which is a necessary condition for the high-frequency maser
emission.
