  The occurrence of low-amplitude flux variations in blazars on hourly
timescales, commonly known as microvariability, is still a widely debated
subject in high-energy astrophysics. Several competing scenarios have been
proposed to explain such occurrences, including various jet plasma
instabilities leading to the formation of shocks, magnetic reconnection sites,
and turbulence. In this letter we present the results of our detailed
investigation of a prominent, five-hour-long optical microflare detected during
recent WEBT campaign in 2014, March 2-6 targeting the blazar 0716+714. After
separating the flaring component from the underlying base emission continuum of
the blazar, we find that the microflare is highly polarized, with the
polarization degree $\sim (40-60)\%$$\pm (2-10)\%$, and the electric vector
position angle $\sim (10 - 20)$deg$\pm (1-8)$deg slightly misaligned with
respect to the position angle of the radio jet. The microflare evolution in the
$(Q,\,U)$ Stokes parameter space exhibits a looping behavior with a
counter-clockwise rotation, meaning polarization degree decreasing with the
flux (but higher in the flux decaying phase), and approximately stable
polarization angle. The overall very high polarization degree of the flare, its
symmetric flux rise and decay profiles, and also its structured evolution in
the $Q-U$ plane, all imply that the observed flux variation corresponds to a
single emission region characterized by a highly ordered magnetic field. As
discussed in the paper, a small-scale but strong shock propagating within the
outflow, and compressing a disordered magnetic field component, provides a
natural, though not unique, interpretation of our findings.
