  Magnetic flares create hot relativistic shocks outside the light cylinder
radius of a magnetised star. Radio emission produced in such a shock or at a
radius smaller than the shock undergoes free-free absorption while passing
through the shocked medium. In this work, we demonstrate that this free-free
absorption can lead to a negative drift in the frequency-time spectra. Whether
it is related to the downward drift pattern observed in fast radio bursts
(FRBs) is unclear. However, if the FRB down drifting is due to this mechanism
then it will be pronounced in those shocks that have isotropic kinetic energies
$\gtrsim10^{44}$ erg. In this model, for an internal shock with a Lorentz
factor $\sim100$, the normalised drift rate $|{\rm DR_{\rm obs}}|/\nu_{\rm
mean}$ is $\sim10^{-2}$ per ms, where $\nu_{\rm mean}$ is the central frequency
of the radio pulses. The corresponding radius of the shocked shell is,
therefore, in the range of $10^{10}$ cm and $10^{11}$ cm. This implies that,
for an outflow consisting of hydrogen ion, the upper limit on the mass of the
relativistic shocks is a few $\times~10^{-10}~M_\odot$, which is considerably
low compared to that ejected from SGR 1806-20 during the 2004 outburst.
