  We show that the absence of the bright optical flashes in most {\it Swift}
Gamma-ray Burst (GRB) afterglows can be explained, if the reverse shock region
is magnetized with a $\sigma \sim 1$, or the emission spectrum of the electrons
accelerated in the mildly magnetized ($0.1<\sigma<1$) reverse shock front is
very soft, or the reverse shock of a non-magnetized outflow is
sub-relativistic, where $\sigma$ is the ratio of the magnetic energy flux to
the particle energy flux. We also find that for $\sigma\gg 1$, the energy
transfer between the magnetized ejecta and the forward shock may be too quick
to account for the shallow decline phase that is well detected in many {\it
Swift} GRB X-ray afterglows.
