We report on observations of flare ribbon kernels during the 2012 August 31 filament eruption. In the 1600 and 304 Å channels of the Atmospheric Imaging Assembly, flare kernels were observed to move along flare ribbons at velocities v ∥ of up to 450 km s-1. Kernel velocities were found to be roughly anticorrelated with strength of the magnetic field. An apparent slipping motion of the flare loops was observed in the 131 Å only for the slowest kernels moving through the strong-B region. In order to interpret the observed relation between B LOS and v ∥, we examined the distribution of the norm N, a quantity closely related to the slippage velocity. We calculated the norm N of the quasi-separatrix layers (QSLs) in MHD model of a solar eruption adapted to the magnetic environment that qualitatively agrees to that of the observed event. We found that both the modeled 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. Contrariwise, lower values of the kernel velocities are seen at the tip of the ribbon hook, where the modeled N is low. Because the modeled distribution of N matches the observed dynamics of kernels, this supports the notion that the kernel motions can be interpreted as a signature of QSL reconnection during the eruption.