  An analysis is presented of the optical polarimetric and multicolour
photometric ($BVRJ$) behaviour of the blazar PKS 2155$-$304 during an outburst
in 2010. This flare develops over roughly 117 days, with a flux doubling time
$\tau \sim 11$ days that increases from blue to red wavelengths. The
polarization angle is initially aligned with the jet axis but rotates by
roughly $90^\circ$ as the flare grows. Two distinct states are evident at low
and high fluxes. Below 18 mJy, the polarization angle takes on a wide range of
values, without any clear relation to the flux. In contrast, there is a
positive correlation between the polarization angle and flux above 18 mJy. The
polarization degree does not display a clear correlation with the flux. We find
that the photopolarimetric behaviour for the high flux state can be attributed
to a variable component with a steady power-law spectral energy distribution
and high optical polarization degree (13.3%). These properties are interpreted
within the shock-in-jet model, which shows that the observed variability can be
explained by a shock that is seen nearly edge-on. Some parameters derived for
the relativistic jet within the shock-in-jet model are: $B=0.06$ G for the
magnetic field, $\delta=22.3$ for the Doppler factor and $\Phi=2.6^\circ$ for
the viewing angle.
