  We report on observations of flare ribbon kernels during the 2012 August 31
filament eruption. In the 1600\,\AA{} and 304\,\AA{} channels of the
Atmospheric Imaging Assembly, flare kernels were observed to move along flare
ribbons at velocities $v_\parallel$ of up to $450$ km\,s$^{-1}$. Kernel
velocities were found to be roughly anti-correlated with strength of the
magnetic field. Apparent slipping motion of flare loops was observed in the
131\,\AA{} only for the slowest kernels moving through strong-$B$ region. In
order to interpret the observed relation between $B_{\text{LOS}}$ and
$v_\parallel$, we examined distribution of the norm $N$, a quantity closely
related to the slippage velocity. We then calculated the norm $N$ of the
quasi-separatrix layers (QSLs) in MHD model of a solar eruption adapted to the
magnetic environment which qualitatively agrees to that of the observed event.
We found that both the modelled $N$ and velocities of kernels reach their
highest values in the same weak-field regions, one located in the curved part
of the ribbon hook and the other in the straight part of the conjugate ribbon
located close to a parasitic polarity. Oppositely, lower values of the kernel
velocities are seen at the tip of the ribbon hook, where the modelled $N$ is
low. Since the modelled distribution of $N$ matches the observed dynamics of
kernels, this supports that the kernel motions can be interpreted as a
signature of QSL reconnection during the eruption.
