One of the challenges in characterization of strain-induced transformations is to create uniform pressure. In this letter, conditions for nearly homogeneous pressure distribution are predicted and achieved experimentally. Compared to hydrostatic loading, plastic shear generally reduces the transformation pressure significantly. We observed, however, an unexpected phenomenon: the transformation of hexagonal to superhard wurtzitic BN under pressure and shear initiated at a pressure comparable to that in hydrostatic compression ($\sim 10$ \ensuremath{{\rm ~GPa}}). In situ X-ray diffraction revealed that plastic shear increases the disorder, while hydrostatic compression does not. This increase neutralizes the transition pressure reduction caused by shear. For the same disorder, shear reduced the transformation pressure significantly, and caused a complete, irreversible transformation.