  Wehrenberg et. al. [Nature 550 496 (2017)] used ultrafast in situ x-ray
diffraction at the LCLS x-ray free-electron laser facility to measure large
lattice rotations resulting from slip and deformation twinning in
shock-compressed laser-driven [110] fibre textured tantalum polycrystal. We
employ a crystal plasticity finite element method model, with slip kinetics
based closely on the isotropic dislocation-based Livermore Multiscale Model
[Barton et. al., J. Appl. Phys. 109 (2011)], to analyse this experiment. We
elucidate the link between the degree of lattice rotation and the kinetics of
plasticity, demonstrating that a transition occurs at shock pressures of
$\sim$27 GPa, between a regime of relatively slow kinetics, resulting in a
balanced pattern of slip system activation and therefore relatively small net
lattice rotation, and a regime of fast kinetics, due to the onset of
nucleation, resulting in a lop-sided pattern of deformation-system activation
and therefore large net lattice rotations. We demonstrate a good fit between
this model and experimental x-ray diffraction data of lattice rotation, and
show that this data is constraining of deformation kinetics.
