When materials are shock compressed, they undergo changes in microstructure that act to relieve the large shear stresses associated with the compression. The plasticity mechanisms that mediate this transition such as slip and twinning remain poorly understood, especially in the case of polycrystals, which make up the majority of real world materials. This work presents a theoretical outline for analysing Debye-Scherrer diffraction experiments under large strains. A method is demonstrated to measure both the components of strain in the normal and transverse directions, as well as crystal orientation using highly textured samples. These theoretical predictions are compared with simulated diffraction patterns from molecular dynamics simulations. This technique is applied to two different experiments on tantalum. The first provides a measurement of the timescale for plastic deformation, which we find similar to comparable experiments in copper, while the second provides the first in situ of observation of twinning in shock compressed metal.